Network system and optical line terminal

ABSTRACT

It is necessary to completely remove the overlapping of signals between plural PONs in order to make the PONs coexist. Accordingly, it is required to share or intensively manage bandwidth use conditions over an optical fiber that serves as a common band between plural systems. Therefore, transmission clocks should be synchronized with high accuracy between the plural systems. A reference clock is provided from an external device or a representative OLT to the entire systems to perform clock synchronization between the plural systems, so that the overall systems are synchronized by synchronizing each OLT with the reference clock. A hierarchical management method is selected that manages ONUs under the control of each OLT by managing band use information arranged for each OLT with respect to an external device or a representative OLT for sharing of bandwidth use conditions between plural systems.

CLAIM OF PRIORITY

The present application claims priority from Japanese patent applicationJP2007-329498 filed on Dec. 21, 2007, the content of which is herebyincorporated by reference into this application.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a system control method that enablesmanaging of ONUs, securing of communication bands, and identifying ofcommunication data in each of plural systems that are disposed to sharean optical fiber when plural optical access devices are installed.

2. Description of the Related Art

Due to the increased demands for broad bands, phone line-based accesstechnologies such as Digital Subscriber Line (DSL) and the like arereplaced by large-capacity access lines using optical fibers asuser-dedicated access lines. Optical access line type is generallyclassified into two types. One type is called “media converter (MC)” andthis is a method of connecting optical fibers from a station of acommunication service provider to a user's home or building to establishoptical fibers in a one-to-one (point-to-point) topology. The other typeof method is called PON (Passive Optical Network), and this provides anoptical coupler (optical splitter) in an optical fiber interval that isdisposed in an interval from a station to a user to split an opticalsignal from a station into 32, 64, or 128 optical fibers. Currently, thePON system, which provides point-to-multipoint topology, has drawnattention for optical access services. One reason is that an opticalfiber from the station to the splitter is shared by all the users in thePON system and therefore costs for establishing lines andmaintenance-related costs may be reduced. The other reason is that thePON system requires a signal multiplexing transmission structure becauseof signals from plural users are arranged and received on the stationside. Multiplexing methods currently adopted include TDMA (Time DivisionMultiple Access), CDMA (Code Division Multiple Access) and so on.Utilizing these functions provides the PON system with an advantage thatmounting optical devices may be done at low costs with respect todevices on the station side.

Currently, there are discussed recommendations (ITU-T Recommendation G.984. 1, “Gigabit-capable Passive Optical Networks (G-PON): Generalcharacteristics”, TTU-T Recommendation G. 984. 2, “Gigabit-capablePassive Optical Networks (G-PON): Physical Media Dependent (PMD) layer”specification) of G-PON (Gigabit-capable PON) for ITU-T (InternationalTelecommunication Union Telecommunication Standardization Sector). Therecommendations have been completed for the main parts, and vendorsstarted to release G-PON products on the market. At the same time,carriers from each country started optical access services that employG-PON. Also, some carriers are providing optical access services whoseinfrastructure is based on GE-PON (Gigabit Ethernet PON) standardized inIEEE (Institute of Electrical and Electric Engineers).

While introduction of optical access systems is accelerated,next-generation optical access systems are already under discussion inFSAN (Full Service Access Network) and IEEE (IEEE 802.3av) that arestandardization groups regarding optical access technologies. In termsof transmission path multiplexing methods, IEEE 802.3av moves towardspeed-up of TDMA and FSAN considers WDMA as another candidate andtherefore both do not reach one conclusion at the current stage.However, both adopt 10 Gbps as a reference respective of downstreamtransmission speed.

Upon high bit rate (broad band) transmission of 10 Gbps, problems withwavelength distortion occur due to S/N deterioration and/or wavelengthdispersion. Accordingly, improving the control of output of a laser orwavelength for suppressing wavelength dispersion is considered as animportant technology. Further, the cost of addressing the problemsshould be in consideration of the market cost not in terms of high-costdevices that is used for a repeating system.

What carriers' value most is not depending on the next-generation PONmethods but the ability to coexist with existing B-PON (Broadband PON,which is standardized in ITU-T in Recommendation G.983.xseries)/GE-PON/G-PON. In the FSAN, it is currently researched to makethe existing PON coexist with next-generation optical access system(NGA) through WDM, and therefore, research is undergoing in terms ofcosts and performance of wavelength filters to be introduced on the ONU(Optical Network Unit; one of the PON equipments located in the usersites) side. It is necessary to perform technological reviews ontechnologies in order to realize 10 Gbps having wavelengths applicableto the NGA restriction. The NGA review group of the FSAN is consideringadopting a wavelength dispersion compensation function for electricalsignals as well as light, a semiconductor optical amp. (such as SOA,EDFA (Erbium Doped Fiber Amplifier), PDFA (Praseodymium Doped FiberAmplifier)), an external modulator (Electro-Absorption ModulatorIntegrated Distributed Feedback Laser; EA-DFB), high-sensitive receiver(Avalanche Photo Diode; APD), and an FEC (Forward Error Correction) torealize 10 Gbps transmission in existing optical fibers (split number of32 or 64, transmission distance of 20 km).

Therefore, multi-system coexistence is also under discussion which iscarried out by time multiplexing using the same wavelength in opticalcharacteristics for a case where plural PON systems coexist. Such asystem construction is expected to be necessary in the future from thepoint of view of costs, which are considered the most important inoptical access.

BRIEF SUMMARY OF THE INVENTION

Costs required to replace transmission devices become significantlyproblematic as FTTH services are being prevalently used. System costsare directly reflected in user's service fees in accordance to the typeof access system. Due to the increase of the speed of DSL and the DSLspeed is migrating toward using FTTH, which in turn acceleratesinfrastructure-providing businesses and exposes price competition.Therefore, cost has recently become an important issue in optical accesssystems and the main topic in respects to standardization. Carriers arerequiring as the development of system exchange or coexistence betweenplural systems having different generation and method be considered inorder to maximally utilize existing resources such as optical fiber aswell as to reduce costs required to establish lines connecting betweenusers' homes and a station of a carrier and device costs.

In the conventional PON, the specifications describing thereof are underthe assumption that the PON exists alone. In a case where plural systemscoexist, it is problematic how to remove interference between thesystems. From the problems of optical characteristics, it has beensuggested to use the same wavelength for plural PONs especially for PONupstream communication, and it has also been suggested to make pluralsystems coexist at the same wavelength by a time-multiplexing methodeven with respect to PON downstream communication.

It is required to remove a signal overlapping between plural PONs inorder to make the plural PONs coexist. Accordingly, it is neededintensively manage bandwidth use conditions over an optical fiber thatserves as a common bandwidth between the plural systems. Thus,transmission clocks should be synchronized with high accuracy betweenplural systems.

A general object of the present invention is to provide a communicationtechnology that enables migration to new systems and enables various PONsystems having different methods to coexist by solving the aboveproblems in a case where an optical fiber is shared by plural PONsystems including a case where each PON has different transmissionspeed.

A first OLT (Optical Line Terminal; one of the PON equipments located inthe carriers' central offices) and a second OLT supply a clock signal toa first ONU and a second ONU, respectively, at common clock timing.

The present invention enables plural PON systems to coexist in order tomaximally utilize existing optical fiber resources in optical accesssystem businesses. Specifically, the present invention may provide acommunication technology that enables compatibility to new systems andenables various PON systems having different methods to coexist.

More specifically, the present invention may completely remove signaloverlapping between PONs due to being capable of sharing or intensivelymanaging bandwidth use conditions over an optical fiber that serves as acommon bandwidth between plural systems by synchronizing transmissionclocks between plural systems.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view illustrating a general system construction in which anoptic fiber is shared by plural PONs that are considered as anapplication of the present invention;

FIG. 2 is a view illustrating an upstream communication control methodusing a time-division multiplexing method that is required to operatethe system shown in FIG. 1;

FIG. 3 is a view illustrating a method of synchronizing operation clocksof an OLT by providing a clock control unit outside the system shown inFIG. 1;

FIG. 4 is a view illustrating a block construction of an OLT-A (1-A) ofthe system shown in FIG. 3;

FIGS. 5A and 5B are block diagrams illustrating a synchronization signalprocessing unit;

FIG. 6 is a block diagram illustrating a synchronization control unit200;

FIG. 7 is a view illustrating a system construction when the systemshown in FIG. 3 has been expanded to strictly adjust the clock timing;

FIG. 8 is a view illustrating a construction of the synchronizationcontrol unit 200 which further includes a feedback function for clockadjustment;

FIG. 9 is a view illustrating a construction example of a table retainedin an adjustment information database shown in FIG. 8;

FIG. 10 is a view illustrating a system construction when thesynchronization control unit 200 shown in FIGS. 5A and 5B has beenprovided in a specific OLT (OLT-A (1-A) herein);

FIG. 11 is a view illustrating a block construction of the OLT-A (1-A)shown in FIG. 10;

FIG. 12 is a view illustrating a system construction when a feedbackline 1200 for clocks is added to the construction shown in FIG. 10;

FIG. 13 is a view illustrating a block construction of the OLT-A (1-A)shown in FIG. 12;

FIG. 14 is a conceptual view illustrating distance difference when eachPON has a different logical distance;

FIG. 15 is a view illustrating transmission timing of a PON upstreamsignal when logical distance has been adjusted in the state shown inFIG. 14;

FIG. 16 is a view illustrating a method of managing a bandwidthallocation condition between OLTs by providing a distance control unit1500 outside the system shown in FIG. 1;

FIG. 17 is a view illustrating a block construction of the OLT-A (1-A)of the system shown in FIG. 16;

FIG. 18 is a view illustrating a block construction of the distancecontrol unit 1500 of the system shown in FIG. 16;

FIGS. 19A and 19B are views illustrating an example of a databaseconstruction including information that should be retained to adjust alogical distance between PON systems as described with reference toFIGS. 15 to 17;

FIG. 20 is a view illustrating a construction example of a table foradjusting an arrival time of the upstream signal between OLTs byintensively managing a response time in all of the OLTs;

FIG. 21 is a view illustrating a system construction when the distancecontrol unit 1500 shown in FIG. 16 that has been provided in a specificOLT (OLT-A (1-A) herein);

FIG. 22 is a view illustrating a block construction of the OLT-A (1-A)shown in FIG. 20;

FIGS. 23A and 23B are views illustrating an example of a databaseconstruction including information that should be retained to adjust alogical distance between PON systems as described with reference toFIGS. 21 and 22;

FIGS. 24A and 24B are views illustrating a construction example of adelay DB 2101;

FIG. 25 is a view illustrating a state in which an upstreamcommunication bandwidth over a shared fiber is time-divided between PONswith respect to the PON-disposed system of FIG. 1;

FIGS. 26A and 26B are views illustrating a construction example of abandwidth allocation table retained in an OLT;

FIG. 27 is a view illustrating a method of managing a bandwidthallocation state between OLTs by providing a DBA control unit outsidethe system shown in FIG. 1;

FIG. 28 is a view illustrating a block construction of a DBA controlunit 2600 of the system shown in FIG. 27;

FIGS. 29A and 29B are views illustrating a construction example of atable for calculating an available bandwidth for each OLT in a DBAcontrol unit;

FIGS. 30A and 30B are views illustrating a construction example of abandwidth allocation table for managing an available bandwidth for eachOLT in a DBA control unit;

FIG. 31 is a view illustrating a construction example of a bandallocation management table in the DBA control unit 2600 when bandwidthallocation to the entire ONUs is determined in a DBA control unit;

FIG. 32 is a flowchart illustrating a processing order of a DBA controlmethod in the system shown in FIG. 27;

FIG. 33 is a view illustrating a system construction when the DBAcontrol unit 2600 shown in FIG. 27 may have been provided in a specificOLT (OLT-A (1-A) herein);

FIG. 34 is a view illustrating a block construction of the OLT-A (1-A)shown in FIG. 33;

FIGS. 35A and 35B are views illustrating a database constructionincluding information that should be retained to autonomously adjust alogical distance between PON systems;

FIG. 36 is a view illustrating a state in which an ONU under the controlof the OLT-A (1-A) is added to the system shown in FIG. 1;

FIG. 37 is a flowchart illustrating a ranging processing part of theOLT-A (1-A);

FIG. 38 is a flowchart illustrating a process in the OLT-A (1-A) fromreceipt of a ranging response to determination of EqD;

FIG. 39 is a flowchart illustrating a process corresponding to a rangingstart notification from another OLT;

FIG. 40 is a view illustrating a bandwidth allocation method(calculation timing) for upstream communication according to a firstmethod;

FIG. 41 is a view illustrating a bandwidth instruction form that may beallocated from an OLT to an ONU;

FIG. 42 is a view illustrating a bandwidth instruction form that may beallocated from an OLT to an ONU;

FIGS. 43A and 43B are views illustrating a construction example of aband allocation information table in case of FIGS. 41 and 42;

FIG. 44 is a view illustrating frame format including allocated upstreambandwidth information notified from an OLT to an ONU in case of FIGS. 41and 42;

FIG. 45 is a functional block diagram illustrating an ONU to perform theband control shown in FIGS. 41 and 42;

FIG. 46 is a view illustrating a bandwidth allocation method upon uplinkcommunication according to the second method;

FIG. 47 is a view illustrating a bandwidth instruction form that may beallocated from an OLT to an ONU;

FIG. 48 is a view illustrating a bandwidth instruction form that may beallocated from an OLT to an ONU;

FIGS. 49A and 49B are views illustrating a construction example of abandwidth allocation information table in case of FIGS. 47 and 48;

FIGS. 50A and 50B are views illustrating a bandwidth use conditionmanagement table retained in a DBA control unit when a DBA period isdifferent in each OLT in the first and second methods;

FIG. 51 is a flowchart illustrating a ranging process;

FIG. 52 is a view illustrating a system construction view when adownstream wavelength is shared and an upstream wavelength is different;

FIG. 53 is a view illustrating a downstream communication method using atime-division multiplexing method;

FIG. 54 is a view illustrating a downstream frame transmission methodwhen a header is shared with respect to the entire downstream periodframes;

FIG. 55 is a view illustrating a downstream frame transmission methodwhen each OLT transmits each downstream frame in the complete form;

FIG. 56 is a view illustrating a sequence of a transmission/receipttiming correcting process between ONU2 s whose upstream transmissiontiming has been changed due to expansion and/or contraction of branchfibers;

FIG. 57 is a flowchart illustrating a process of an OLT when all of thePONs uses a common wavelength with respect to upstream and downstream;

FIG. 58 is a view illustrating a structure of the clock synchronizationcontrol unit for a clock synchronization method when a downlinkwavelength is shared;

FIG. 59 is a view illustrating an example of a signal transmitted whenthe phase of a clock is extracted based on strength of the signal; and

FIG. 60 is a block diagram illustrating an ONU when downstreamwavelengths are the same.

DETAILED DESCRIPTION OF THE INVENTION

The object of the present invention aims to minimally suppress aninfluence on the other systems and an influence on data forcommunication, which take place when an optical fiber is shared byplural PON systems and is caused by a ranging process in a certainsystem. Therefore, a mechanism has been achieved in which PON sectioncommunication timing of each system is shared between PON systems thatshare an optical fiber. This enables efficient operation of the pluralPONs while maintaining basic functions of PON. Plural variations may beconsidered in the mode of disposing the PONs but the present inventionmay apply to all of the configurations. Plural variations are consideredwhen plural PONs are disposed in the form of sharing an optical fiber.An example includes a case where the plural PONs use a common upstreamwavelength or a common downstream wavelength. For a system configured insuch an organization, four methods are generally considered according tocombinations of wavelengths used for upstream communication anddownstream communication.

When each of the disposed PONs uses different wavelengths with respectto each of upstream communication and downstream communication, the sameoperating method as the prior art (existing recommendations) may applyto the system. It is problematic when one of the uplink communicationand downstream communication uses a common wavelength.

For example, it is difficult to receive a correct signal at the OLT sidewhen the upstream wavelength is shared because signals from all of theONUs coexist on the optical fiber after the signals from the plural ONUshave been multiplexed (that is, on the optical fiber disposed at the OLTside to transmit a multiplexed signal). The control of communicationtime, which has been conventionally performed for each and every ONU inan individual PON system, should be generally carried out for the all ofthe ONUs that share the optical fiber in order to avoid this problem. Inthis case, all of the ONUs that use the wavelength for the upstreamcommunication are subjected to controlling communication time withoutdepending on the OLT to which the ONUs belong. The communication timingcontrol should be carried out on the OLT side since time-multiplexingcommunication should be performed between the ONUs.

Moreover, when a downstream wavelength is shared, the plural PON systemsshare the head information of a downstream communication frame. In thiscase, therefore, each OLT does not independently decide timing when aninstruction is made (a frame is transmitted) to the ONU that issubordinate to the OLT and should control the ONU under its managementwhile always adjusting the frame transmission timing together with theother OLTs that are connected to the same optical fiber. At this time,timing when an upstream communication bandwidth is notified to the ONUshould be interlocked between all of the PONs. Therefore, the bandwidthinstruction timing (Dynamic Bandwidth Assignment (DBA) processing timingand band control period) from the OLT should be operated under anysynchronized condition even if each PON uses its own individualwavelength for the upstream communication. That is, the communicationtiming needs to be controlled on the OLT side to perform timemultiplexing communication of PON control signals.

As mentioned above, in either case of the upstream wavelength or thedownstream wavelength, coexistence requires the control of the frametransmission timing in the PON section. In the PON, the frametransmission timing is managed by the OLT side no matter whether it isupstream or downstream. Therefore, the means becomes important, whichshares bandwidth information and communication timing (clock)information between the plural OLTs. Generally, there are considered asystem having a function of managing communication timing information ofeach OLT outside the OLT and a system in which each OLT has a controlfunction to manage other co-existed OLTs' timing control information.Moreover, there are adopted two methods as a bandwidth (timing) controlmethod in these systems. That is, there exist a centralized controlmethod that independently instructs the other OLTs with thecommunication timing by either an external timing information managementunit in the former system or the OLT in the latter system and adistributed processing method that determines the timing by each OLT andshares the result between the OLTs.

In exemplary embodiments of the present invention, communication rate ofan individual PON or type of communication protocols used in the PONsection does not matter. For example, a combination of G-PONstandardized in ITU-T and GE-PON, G-PON, or next generation PON (forexample, ITU-T based 10 Gbps-PON, IEEE 802.3av based 10 Gbps E-PON, orthe like) may be available. A frame configuration of ITU-T based G-PONis assumed as an example in the following description.

According to an exemplary embodiment of the present invention, a systemwith a common upstream wavelength will be first described herein as anexample, which has a timing information management unit outside an OLT,wherein the timing information management unit independently suppliesclocks to the OLT. Thereafter, exemplary embodiments and effects will bedescribed with respect to a case where the system has a downstreamwavelength and a case where the system has common upstream anddownstream wavelengths.

FIG. 1 is a view illustrating a general system construction in which anoptical fiber is shared by plural PONs that are considered as anapplication of the present invention. An OLT-A (1-A) controls NA ONUsfrom ONU-A-1 to ONU-A-NA, and performs start of ONUs, bandwidth controlof the ONUs by a DBA, and state management of the ONUs. An OLT-B (1-B)controls NB ONUs from ONU-B-1 to ONU-A-NB, and performs start of ONUs,bandwidth control of the ONUs, and state management of the ONUs. TheOLT-A and the OLT-B share an optic fiber 100 in a PON section (OpticalDistribution Network (ODN) section). The OLT-A and the OLT-B areconnected to the shared optical fiber 100 through optic fibers 111-A and111-B for OLT connection, respectively. The optical fibers for OLTconnection are connected to the optical fiber 100 through a wavelengthdivision multiplexer (WDM) 120. The WDM 120 has a function ofmultiplexing a signal transmitted from the OLT-A to the ONU and a signaltransmitted from the OLT-B to the ONU.

In this system, signals transmitted from the OLT-A to the ONUs under itscontrol, respectively, use different wavelengths from each other(hereinafter, a signal directed from the OLT to the ONUs is referred toas “downstream signal” or “downstream communication” and a signaldirected in the opposite direction of the downstream signal is referredto as “upstream signal” or “upstream communication”). In the drawings,the downstream signals are indicated as λd_A and λd_B. For upstreamsignals, ONUs transmit the signals to the OLT at the same wavelength nomatter whether they are ONUs under the control of the OLT-A or ONUsunder the control of the OLT-B. These upstream signals are indicated asλu in the drawings. Accordingly, the upstream signals from the entireONUs are distributed to the OLT-A and the OLT-B with the same strength.The signals multiplexed by the WDM 120 pass through branch line opticalfibers 101-A-1 to 101-A-NA and 101-B-1 to 101-B-NB, each of which isconnected to the individual ONU, via an optical splitter 150 (whichfunctions as an optical coupler for the upstream signals) to be sent tothe ONUs. Light distributed omni-directionally by the optical splitterarrives the entire ONUs connected to the optical splitter no matterwhether they are under control of OLT-A or OLT-B. Accordingly, the ONUshave their own wavelength block filters 110-A-1 to 110-A-NA and 110-B-1to 110-B-NB therein to identify the signals from the OLT to which theybelong. The wavelength block filter is designed to screen light havingwavelengths that the filter does not receive so that unnecessary lightis not mixed at an optical receiver (not shown herein) located behindthe wavelength block filter. A communication path 130-A from the OLT-Ato the external and a communication path 130-B from the OLT-B to theexternal are a line used for connecting to, for example, a local IPnetwork that is an upper network of an optical access line, and thesemay employ Ethernet (registered trademark), SONET/SDH, and the like asone line type. The type of line and communication protocol between theOLTs and the upper network are not particularly restricted in thepresent invention. Lines 120-A-1 to 120-A-NA and 120-B-1 to 120-B-NB,which are located downstream of the ONUs, are considered to be connectedto LANs of a building or home. For this case, Ethernet or telephone lineis primarily considered to be used as the line. Of course, the type ofline and communication protocol is not particularly restricted herein,either.

A mode in which the optic fiber 100 is shared by further plural OLTs isconsidered. The description has been made using the OLT-A and the OLT-Bin this exemplary embodiment. Even though the number of OLTs connected,that is, the number of PONs sharing the optical fiber 100, increases, itmay be similarly applied to the present invention without losing thefeatures of the present invention. For this case, OLT connection opticalfibers 112 and 113 are used for connection with other OLTs. Further,branch line optical fibers 102 and 103 are used to add or move ONUs tobe managed by the above-mentioned new OLT or the ONUs to be managed bythe OLT-A or OLT-B.

FIG. 2 depicts an upstream communication control method using a timedivision multiplexing method, which is necessary to operate the systemshown in FIG. 1. In the system shown in FIG. 1, it is impossible foreach OLT on the receiving side to identify the signals transmitted fromthe ONUs under its control by the wavelength because all of the ONUs usethe same wavelength with respect to the upstream signals from the ONUs.The conventional PON refers to the transmission/receipt timing by a DBAfunction in response to a communication bandwidth request from the ONUunder its control to identify the ONU, which is the transmission source,from signals having the same wavelength. The time division multiplexing(TDM) method is also used herein to extract the signal from theindividual ONU. The OLT has managed only the communication situationwith the ONU under its control when allocating a communication time toan individual ONU because the conventional PON individually existedwithout sharing the optical fiber. Therefore, it is possible in thissystem for an OLT to avoid the upstream communication time used by theother OLTs so as to prevent overlapping of signals when the OLT performsa bandwidth control.

FIG. 2 is a view illustrating a frame multiplexing method over anoptical fiber upon upstream frame transmission from an ONU 2 to an OLT1. This shows a situation in which a frame is transmitted from the leftside of FIG. 2 to the OLT 1 on the right side of FIG. 2. Further, FIG. 2shows an example of an arrangement situation for a frame whosetransmission time is late from the ONU 2 toward the left side, whereinthe right side of FIG. 2 shows the earliest data transmitted. The dottedline indicates a basic frame period (for example, 125 microseconds).

The frames transmitted from the ONU 2's respectively are multiplexed toa single optical fiber when passing through the splitter 3. In thedrawing, 1401-1 to 1401-n refer to the transmission location and thesize of fixed bandwidth communication data transmitted from the ONU2#1to ONU2#n, respectively. The data 1401-1 to n, which have beendistributed over the plural optical fibers before passing through thesplitter, are multiplexed after having passed through the splitter. Theframes 1402 to 1407 refer to variable bandwidth data transmitted fromthe ONU 2's. The variable bandwidth data are inserted not to overlap thefixed bandwidth data upon multiplexing by a DBA mechanism.

There is an associated operation that is necessary between OLTs toenable the optical fiber 100 to be shared as described above. Furtherthereto, there are required (1) a function of synchronizing clocksbetween the OLTs, (2) a function of sharing a use situation of upstreambandwidths between the OLTs, and (3) a function of cooperating with theDBA control.

There are a method of supplying a reference clock, which is common withrespect to the entire systems, from the outside of the OLT, and a methodof supplying a reference clock by a representative OLT in order toachieve function (1). Also, there are a method of providing a commondatabase at the outside so that the entire OLTs may refer to it and amethod of providing each OLT with an individual database to let thebandwidth use situation notified between the OLTs in order to achievefunctions (2) and (3).

Below, clock synchronization will be described. FIG. 3 is a viewillustrating a method of synchronizing an operation clock of an OLT byproviding a synchronization control unit for clock synchronizationoutside the system shown in FIG. 1. The synchronization control unit 200is provided behind the OLTs and the OLTs are connected to thesynchronization control unit 200 through lines 201-A, 201-B, 202, 203,and 210 for clock supply. A clock generating unit 830 (refer to FIG. 6)is included in the synchronization control unit 200. A synchronizationsignal processing unit 670 (refer to FIGS. 5A and 5B) is included in theOLT. The synchronization signal processing unit 670 synchronizes a clockgiven by a signal from the clock generating unit 830 and performs asignal process in the PON section according to the extracted clock.

Plural supply methods are considered for clocks to be supplied from thesynchronization control unit 200 to an OLT with required accuracy. Thefastest clock among the clocks in the PON section should be used toperform synchronization within accuracy of 1 bit between the pluralsystems in order to improve the communication efficiency in the PONsection. Practicably, it is preferable to perform an operation with theclock that corresponds to the least common multiple of the clocks ofeach PON system. When PON interfaces are used in the systems sharing theoptical fiber wherein the clock speed of one PON interface correspondsto an integer multiple of that of another one, for example, like whenany one PON interface has the clock speed of 10 Gbps and another PONinterface has the clock speed of 2.5 Gbps (corresponding to a case ofG-PON), the PON interface having the fastest clock speed is used amongthe disposed PON interfaces. It is possible to make upstream anddownstream transmission timing of a low-speed clock consistent to thatof a high-speed clock by matching the clock speed to the high-speedside. This enables controlling the timing of variation in opticalsignals with respect of all of the PON interfaces to be disposed. On thecontrary, matching the clock speed to the low-speed side may not detectthe timing of variation in some optical signals in the PON interfaceswhich operate at a high-speed clock. Accordingly, density of data thatmay be transferred is correspondingly lowered.

On the contrary, moreover, it is also practically possible to notify theclock that is sub-sampled at a smaller bit rate than the bit rate in thePON section (for example, by 1/n) when a long guard time under the startof the upstream frame transmission is occurring while there is room inaccordance to the upstream band availability of the PON section. Thismethod may be used, for example, in a case where high bit rate PONinterfaces are disposed. Because of having a high transmission speed,this method may transmit a relatively sufficient amount of data at a TDMAccess (TDMA) guard time. Setting the guard bit broadly has thefollowing merits. For example, in case of being transmitted by the TDMAsignals, fast data is transmitted from each ONU with burstcharacteristics, however; it is difficult to clearly establishsynchronization of the optical signal and frame at the opticaltermination, which is on the OLT side. Because data is at high-speed,the noises detected by a photo-detector are expanded, and problems occurin terms of technologies and costs to detect the signal patterns at sucha speed in order to be capable of following the high-speed bit rate toextract the data. For example, it is possible to be co-existent at thelower-speed side than the optical signal rate using an existingphoto-detector by selecting a clock corresponding to the greatest commondivisor of operation clocks of disposed PON interfaces. For example, ithas been required in the existing G-PON to catch a relatively short-termsignal (within the receipt of restricted bit number) when receiving anupstream frame at the OLT side. Such restriction in a bit number isdifficult and vendors owning technologies that may be realized arelimited, especially taking into consideration operation speed of opticalparts. Therefore, current recommendations (ITU-T Recommendation G. 984.3, “Gigabit-capable Passive Optical Networks (GPON)=Transmissionconvergence layer specification”) mitigate conditions, so that framecapturing time may be adapted to be lengthened upon startup of ONUs. Assuch, it may lead to merits such as reduction in costs of thephoto-detector and realize stable operations to expand the guard bitswhile considering the limitations of performance of a part whichreceives optical signals, that is, optical devices and SERDES(SERializer DESerializer). As mentioned above, it is natural to dependon objects of system establishment either to supply high-speed clocks orlow-speed clocks.

Further, although not relating directly to the clock synchronizationitself, a reference clock for transmitting and receiving the upstreamand downstream frames (125 microseconds period, i.e. 8 KHz clock inG-PON) needs to be supplied to each PON system together with the clocksignal in order to perform the frame process as described above. Thiswill be described in association with the control of DBA to be describedlater. FIG. 4 depicts a block diagram of an OLT-A (1-A) in the systemshown in FIG. 3. The OLT-A (1-A) has an interface with an optical fiber111-A at the PON section side (Access Network Interface (ANI) side). WDM660 plays a role as the interface herein. The WDM 660 is used toseparate the wavelengths of the upstream signal and the downstreamsignal, and there is separately prepared another WDM 120 to connectbetween OLTs in FIGS. 5A and 5B. Ethernet (registered trademark), 10Gbps Ethernet (registered trademark), TDM interfaces, whoserepresentative example includes T1 and E1, and the like may be used asan interface 610 at the carrier network side (Service Node Interface(SNI) side) of the OLT-A (1-A).

The process of downstream signals of the OLT-A (1-A) has the followingflow, for example, when the Ethernet (registered trademark) is used asthe interface. First of all, if a signal is inputted to interfaces 610-1to 610-n, the inputted signal performs a synchronization process in theinterface 610 to terminate the protocol. A receipt processing unit 621determines the destination of the downstream frame in the PON section onthe basis of the head information of a received downstream frame. Thatis, it is determined whether the destination information is locatedbefore a specific ONU or the plural ONUs, or the destination is suchthat information should be processed by OLT termination (that should betransferred to CPU included in a PON control unit of the OLT). Further,the receiving processing unit 621, if necessary, performs conversion,provision, and deletion of address information in response to the typeof destination information or receiving data. Here, the destinationinformation includes logic path information, such as VLAN tags or MPLSlabels, as well as the routing information of MAC and IP beginning. Theinformation that should be transferred to the CPU is transmitted to thePON control unit 600 and the data to be transferred to the ONU istransmitted to the downstream frame generating unit 622. The downstreamframe generating unit 622 makes a frame for downstream transmissionbased on the frame received from the receipt processing unit 621 and theinformation from the PON control unit 600. The information from the PONcontrol unit 600 includes DBA information to be notified to the ONU andcontrol information for performing control/management in the PON section(for example, Physical Layer Operations, Administration and Management(PLOAM) messages and the like in G. 984.3). The control information isgenerally inserted into the header of the downstream frame, however, maybe stored in the payload of the frame in a case where the ONU may berecognized with respect to a special usage such as a message unique to avendor (frame format is not limited in the present invention). Atransmission processing unit 623 buffers the frame generated by thedownstream frame generating unit 622, and reads it according to thepriority of frame information, conditions of the destination ONU, andprocessing performance. And, an E/O converting unit 631 converts theread result into an optical signal, and transmits the converted resultaccording to the transmission clock.

In connection with the upstream signals, the following steps areoperated. When an uplink wavelength signal separated by the WDM 660 isreceived by an O/E converting unit 632, serial data is reproduced basedon the received signal. The transmission clock of the upstream signal isextracted based on the reproduced signal. The receiving data isserial/parallel converted according to the reproduced clock, and thenframe synchronization is performed with respect to the upstream frame.At this time, the reproduced clock is received at the timing designatedupon transmission of the downstream frame with respect to the ONU beforethe upstream frame is received by the OLT-A (1-A). The designated timingis retained in DBA information A in the PON control unit 600 of theOLT-A (1-A) until the upstream frame is received to identify whether theupstream frame is received at the right timing upon receipt. This isdone by comparing and referring to the received clock information andDBA information 602 with respect to the receiving clock identifying unit652. It is very important that the reference distance measurement iswithin the ranging upon startup of the ONU for the phase surveillance ofthe upstream frame. In ranging, the distance of the PON section (roundtrip communication time in the PON section) is measured on a per-one bitbasis. Therefore, when different-speed PONs are disposed, the one-bitdifference causes a time difference (difference in receipt timing) often times in case of 1 Gbps to 10 Gbps and four times in case of 2.5Gbps to 10 Gbps. Accordingly, the different-speed disposed systemsrequire a mechanism of improving accuracy between the reference distanceand a mechanism of suppressing the phase variation under operation atthe early distance measuring stage (that is, ranging). This rangingreference location (measurement start location) is determined inprocessing an electrical signal in the OLT.

From a point of view of when plural optical modules (that is, pluralOLTs) are provided, each of the plural optical modules has a slightdifference in processing time upon E/O conversion, strictly speaking, itis preferable to make the optical modules uniformed. However, it isdifficult to make this environment when OLTs are additively disposed,and therefore, the following descriptions will be made assuming thateach optical module is individually provided to each OLT. Furthermore,it is necessary to make the ranging reference points consistent in orderto synchronize the clocks of the optical signals as much as possible.Even in a case where downstream wavelengths are different from eachother, it is preferable to make the start timing of the rangingconsistent in each OLT to perform an exact ranging. Further, stricteradjustment is required when different-speed PONs are disposed. Thetreatment of this case will be described later (FIG. 8).

At this point, when the timing is different from the expected one, EqDinformation A601 retained in the PON control unit 600 is updated and theEqD information for each ONU is inserted into the header of thedownstream frame to correct the logical distance. The receipt processingunit 641 identifies whether it is data that should be transmitted to theSNI or the PON control unit 600 on the basis of the header informationof the receiving frame like in the process of the downstream frame.Besides, addition, conversion, and deletion of information on thedestination are performed according to the setup of path information onthe basis of the header information of the receiving frame. The upstreamframe generating unit 642 generates the header and payload of the frameto be transmitted to the SNI from the information of the PON controlunit 600 and the receipt processing unit 641, and transmits them to thetransmission processing unit 643. The transmission processing unit 643buffers the generated frame, reads it according to the priority of theframe information, the condition of the destination ONU, and processingperformance, and transmits the data through the SNI-sided interface 610.

The interface 680 with the synchronization control unit 200 is aninterface to receive a clock that is generated and supplied by thesynchronization control unit 200. The clock is sent to thesynchronization signal processing unit 670, which in turn generatestiming based on the received clock. The clock obtainable herein istransferred to the clock adjusting unit 651, which in turn transmits adownstream signal according to the clock. The clock adjusting unit 651has a function of managing the difference in clock phase between thedownstream signal and the upstream signal. The conventional exchangerconstituting a PON and a telephony network has employed a configurationof receiving a reference clock from a base station of a carrier throughthe clock distribution network. At this time, the received clock isstored in the device at a PLL (Phase Locked Loop) to be used as a deviceclock. Further, only the clock generated by a crystal oscillator in thedevice may be operated. In a case where plural PONs are disposed, use ofexternal clocks or clock transfer (FIG. 10) from the reference device isinevitable for cooperation between devices. In the conventional method,occurrence of the device synchronization has been sufficient at amillisecond level, for example, in the telephone network, however, phasesynchronization of clocks is important for DBA cooperation or ranging tobe described later, which requires synchronization of within about 1 bit(sub-microseconds) in case of PONs. Functions and effects necessary forthis will be described below (FIG. 8).

In addition, the configuration of the synchronization control unit shownin FIG. 3 is equal to the configuration of the device shown in FIG. 8,which will be described below, when the feedback function is omitted.The system shown in FIG. 3, which has been described so far, isnecessary when the same optical fiber is shared by plural PON systemswhich have a common laser wavelength used in at least one of theupstream communication and the downstream communication. A common clockis provided to all of the disposed systems with respect to the OLTs,which are devices located at a side of controlling the upstream anddownstream data transmission timing in the PON sector in order to avoidoverlapping of signals in the PON sector. It can be possible toexternally supply a common clock with respect to the overall OLTs byproviding the synchronization control unit 200. Use of the supplyingclock as reference information for adjusting the operation timingbetween the OLTs may control the DBA processing period and frametransmission and receipt timing in both directions of the PON section.

Even in a case where downstream signals have different wavelengths, itis necessary to exactly adjust the reference point of the ranging set inthe OLT, and the clock synchronization and the above-mentioned 8 kHztiming synchronization are necessary to measure the communicationdistance during the ranging as well as to avoid overlapping betweenoptical signals upon transmission of the frame.

FIGS. 5A and 5B are block diagrams of the synchronization signalprocessing unit 670 included in the OLT 1 shown in FIG. 4. FIG. 5A is ablock diagram when a clock from the synchronization control unit 200 isused as the operation clock of the OLT without providing any referenceoscillator to the inside of the OLT, and FIG. 5B is a block diagram wheneach oscillator is provided to the inside of each OLT so that clocks forcontrolling the PON section are acquired by receiving a clock signalfrom the synchronization control unit 200 and mapping the received clocksignal with the clock in the device.

In FIG. 5A, a clock signal is received from the synchronization controlunit 200 by the clock receiving unit 680 that functions as a clockreceiving interface and the received clock signal is transmitted to theOLT 1 as it is over a line 701. In this case, there is provided a singleclock oscillator for the entire system, and this may achievesynchronization for the entire system. Accordingly, a stable operationmay be done without any shift in clocks between the OLTs. It may bepossible to provide a function 702 of monitoring the receiving clocks inthe block.

FIG. 5B depicts a constructional example of the synchronization signalprocessing unit 670 in a case where each clock oscillator is provided inthe inside of each OLT. The synchronization signal processing unit 670employs a oscillator 717, a phase/frequency detecting unit 716, a LowPass Filter (LPF) 712, and a Voltage Controlled Oscillator (VCO) 711 togenerate the OLT control clock. The clock signal from the oscillator 717passes through the phase/frequency detecting unit 716, the LPF 712, andthe VCO 711 and transmits to the OLT. Meanwhile, the clock signal fromthe synchronization control unit 200 is sent from the clock receivingunit 680 to the phase/frequency detecting unit 716. This signal passesthrough the LPF 712 and the VCO 711, similar to the signal from theoscillator 717, and transmits to the OLT. The clock receiving unit 680includes a clock detector 715 to synchronize the clocks from these twosystems (synchronize the clock inside the OLT with the clock from thesynchronization control unit 200). The clock detector 715 has a functionof detecting the clock timing of both clocks to adjust it so that theinternal clock is synchronized with the reference frequency. Further,there is an effect of stabilizing the clock supplied in the OLT byperforming a loop-back on the individual clock from these two systemsfrom the VCO 711 to the clock detector 715. This operation may berealized by providing a dividing circuit and a comparison circuit in theclock detector 715. Further, the operation frequency may be fixed byproviding a clock detection circuit in the clock detector 715, and thismay maintain the operation with some accuracy until the clocks, whichmay have been lost for a period of time, are recovered as well asprevious clock timing upon startup.

FIG. 6 is a functional block diagram illustrating a function of thesynchronization control unit 200 shown in FIG. 3. A clock signalgenerated by the oscillator 805 is outputted to the clock generatingunit 830. A general PLL circuit construction is described herein as anexample of the clock generating unit 830. The clock generating unit 830includes a phase/frequency detecting unit 803, an LPF 802, a VCO 801,and a dividing circuit 804. A clock signal generated by the clockgenerating unit 830 passes through the phase/frequency detecting unit803, the LPF 802, and the VCO 801 and is sent to the clock transmittingunits 810-1 to 810-n. The signal which passes through the VCO 801 and isthen sent to the clock transmitting unit 810 is also sent to thedividing unit 804 at the same time. The dividing circuit converts theinputted clock by the factor of 1/n and transmits the result to thephase and the phase/frequency detecting unit 803 as a comparative clock.The phase/frequency detecting unit 803 compares the clock from theoscillator 805 with the clock from the dividing circuit 804 tosynchronize the clock for transmission having a period of 1/n, which isgenerated by the dividing circuit 804, with a stable clock of the stableoscillator 805. The stable clock obtained is supplied from the clocktransmitting unit 810 to an external device.

FIG. 7 depicts a system construction when the system shown in FIG. 3 hasbeen further expanded so that the timing is further strictly adjusted.If the PON system sharing the optical fiber 100 has a bit rate of 1 Gbpsto a few Gbps of the conventional GE-PON or G-PON, about 1 nanosecond isrequired to make the clock synchronized with an accuracy of 1 bit, andif the PON system is IEEE 802.3av (10 Gbps E-PON) or 10 Gbps PON ofITU-T, which is currently discussed to be the standard, control isneeded to be performed with an accuracy of more than about 100picoseconds with respect to the clock synchronization at the bit level.In order to control under high accuracy, a structure that performs fineadjustment while viewing the state of the transmitted clock as well astransmission of the clock from the synchronization control unit 200 toall of the ONUs is required. FIG. 7 illustrates an aspect in which asignal line 220 for feedback is added as a line for clock monitoring toperform fine adjustment. This function is required to use the systemdescribed above with reference to FIG. 3 in a case where the receivingtiming of the uplink signal as well as the downstream signal desire tobe strictly adjusted. For example, all of the PON-IFs sharing theoptical fiber are operated at the same clock when the upstream bandsdesire to be maximally utilized. As seen from the OLT, it is required toinstruct the transmission timing within an accuracy of 1 bit by anoptical signal with respect to all of the ONUs. At this time, eventhough clock synchronization is needed at the level of 1 ns, clockcontrol is generally difficult to perform with such high accuracy due toinconsistency of signal processing time at the PON-IF or expansion andcontraction of the optical fiber when DBA notification is conducted fromthe OLT to the ONU on the basis of the conventional ranging function.Most of all, it may be said that it is actually impossible to make theclocks comply with the high-speed clock with respect to the operationclock between the OLTs (PON system) because the processing time isdifferent for each and every OLT. Accordingly, fast clocksynchronization may be achieved by actually monitoring the communicationclock of each PON to correct the errors with the feedback circuit asintroduced in the exemplary embodiment.

As a consequence, the synchronization control unit 200 is changed tohave the feedback function as well as the function of clock generationand notification. FIG. 8 depicts a construction of the synchronizationcontrol unit 200 which further includes a feedback function. The clocksignal generated by the clock generating unit 830 is transmitted from ncinterfaces which are interfaces for clock signals (810-1 to 810-nc) tonc OLTs. The clock is transmitted through lines 201-1 to 201-nc, each ofwhich connects each OLT to the synchronization control unit 200.Meanwhile, the clock transmitted in the PON section from the OLT-A(1-A), the OLT-B (1-B), and the other OLTs, is returned to thesynchronization control unit 200 by the lines 220 for feedback at thetime of having passed the WDM 120. The feedback signal inputted from thelines 220 shown in FIG. 8, which has plural downstream wavelengths, issplit by the WDM 850 for each and every wavelength, and then O/Econverted with respect to the clock generating units 820-1 to 820-nw sothat the serial data is extracted. The operation for stabilizing theclock of the clock reproduction unit 820 is identical to that describedabove with reference to FIGS. 5 and 6, and therefore repetitivedescriptions will be omitted. The clock signal extracted from the serialdata of the clock reproduction unit 820 is transmitted to the clockcomparing unit 840. The clock comparing unit 840 detects the differencebetween the transmitted clock signal and the reference clock, and theclock correcting unit 842 determines the amount of correction for eachand every clock lines 810-1 to 810-nc. The corrected value obtainedtherein is notified from the clock correcting unit 842 to each of theinterfaces 810-1 to 810-nc, and the correction-processed clock isoutputted to each clock line 201 by each interface, respectively. In acase where control is conducted with high accuracy, there is highpossibility that each clock transmitted from each OLT in the PON sectionslightly varies its timing when the clock is unilaterally transmittedfrom the synchronization control unit 200. This is why it is difficultto completely synchronize the signal from each device due to the delayin processing of each block in the OLT or difference in clocktransmission time in the OLT circuit, which occurs when different typesof PONs are disposed. Utilization of the feedback mechanism shown in theexemplary embodiment may identify the degree of a shift in the signalsactually transmitted in the PON section, so that influence-correctedsignal may be transmitted. Accordingly, it may be possible to finelyadjust the transmitted clock.

The shift correction completes an important role in association with theranging process. Because the reference point of the ranging isdetermined by each OLT, however, the reference point of the ranging atthe startup time of the OLT is thereafter treated as a reference pointunder operation, time (timing) when a ranging request is transmittedfrom each OLT to the ONU under the control is required to be as accurateas possible for a stable operation. Further, the amount of variation intransmission delay due to expansion and/or contraction of the opticalfiber needs to be assumed on the order of a few nanoseconds. When thereis the shift of 1 bit in the reference point, the shift of the roundtrip time observed when the PONs having the same bit rate are disposedis only on the order of a few bits. Upon transmission of upstream frame,a guard time is placed in the upstream signal taking into considerationa shift of the response timing from the ONU and time necessary forstartup of a laser for transmitting an upstream signal (a few to a fewtens of bits as a current bit value), so that this variation may beabsorbed. Of course, in case of high bit rate, time interval per bandcontrol unit (1 bit) is minute, and therefore, fine control is requiredusing the feedback line for control with the same bit numbers.

Clock adjustment upon initiation of the ranging is necessary, inparticular, when PONs having different speeds (for example, PONs whosecommunication speeds are 10 Gbps and 2.5 Gbps, respectively) aredisposed. The number of bits that may be transmitted at 10 Gbps in theunit time is four times of 2.5 Gbps. Accordingly, in a case where theclock synchronization is conducted with accuracy at the low bit rateside, a time shift corresponding to one bit of clock has an influence of4 bits when being recalculated as the clock at the high speed side.Generally, even though the guard band is placed considering thevariation in the reference point of the ranging (that is, an error ofthe set EqD), more guard bands than necessary should be prepared totransmit high bit rate signals, and this may lead to lowering bandavailability.

In order to achieve synchronization with the clock at the high speedside, the PONs having different speeds are connected to avoid the aboveproblems. Hereinafter, specific examples are described with respect to acase of G-PON. High speed clock is mixed at the location where low speedclock is shifted to conduct transmission and receipt of data in a casewhere PONs having different speeds coexist. That is, the bit locationrepresenting the data (such as boundary of 8 KHz frame) should beadjusted by shifting the high-speed clock (in case of 10 Gbps and 2.5Gbps) by a bit number of four times. However, it reduces the differencebetween transmission and receipt phases of low-speed bit rate upondownstream frame transmission to send high-speed clocks to PONsoperating at low speed to generate 2.5 Gbp clock phase so that theclocks primarily synchronized at a high-speed bit level become ¼ times,thus making it possible to determine the ranging reference location withgood accuracy.

Further, the construction of the synchronization control unit 200 in thesystem shown in FIG. 6 includes a clock generating unit 830 andinterfaces 810-1 to 810-nc among the construction blocks shown in FIG.8, CPU for controlling the clock generating unit 830 and the interfaces810-1 to 810-nc, a memory, and a control interface.

A method may be considered that retains a table in an output adjustingunit shown in FIG. 8 as an information database for adjusting clockphase. FIG. 9 depicts a construction example of a table retained in theadjustment information database shown in FIG. 8. A difference 902 fromthe reference clock and a corrected value 903 upon transmission withrespect to the reference clock, which are calculated for each and everyline ID 901 that shows a clock transmission line 810, are retainedcorrespondingly. Since the clock is always fed back, the table may berewritten whenever the table has any variation. Specifically, when a PLLis used as each OLT with respect to the external clock, an error mayoccur on the order of one to a few clocks at the time when the opticalsignal is transmitted. Here, it is preferable to use the clock as it iswhich is notified from the synchronization control unit to each OLTwithout the PLL to perform high-bit rate data transmission (see FIG.5A). In a case where a specific OLT becomes the master to control theclock, it is preferable that the clock is extracted upon transmission ofthe optical signal in the master OLT and the extracted clock is notifiedto the other disposed OLTs. The OLTs that have received the clock usethe clock without the PLL, as the in-device clock, as it is.

FIG. 10 depicts a system construction where the synchronization controlunit 200 shown in FIG. 3 is provided in a specific OLT (OLT-A (1-A)herein). Providing the synchronization control unit 200 in the OLT-Aenables the entire PON system to be made compact compared to a casewhere the synchronization control unit 200 is positioned outside theOLT-A. This allows the size of the circuit for controlling the PONsystem to be reduced, thus making it possible to lower costs. Also,integration of the control function prevents inconsistent efficiency ofthe transmission process or external noises that may be caused uponcommunication with external devices, thus making it possible to improveaccuracy of control. In particular, this effect may be anticipated in acase where this function is mounted in the OLT which operates at themaximal bit rate among the disposed PONs.

In this case, the function of the synchronization control unit 250 isidentical to that provided outside as shown in FIG. 3. The OLT-A (1-A)generates a clock through its own frequency oscillator. The OLT-A (1-A)has a function of transferring its own clock to other OLTs like thesynchronization control unit 200 shown in FIG. 3, in addition to afunction of communicating clock information with the ONU through thedownlink signal. The device construction of the other OLTs besides theOLT-A (1-A) is as shown in FIG. 4.

FIG. 11 shows a block construction of the OLT-A (1-A) shown in FIG. 10.In this case, the synchronization signal processing unit 670 shown inFIG. 4 is replaced with a synchronization control unit 1100. A clockgenerating unit 1101 inside the synchronization processing unit 1100 hasa function as shown in FIG. 8. A timing generating unit 1102 determinestiming information to be notified to other OLTs with respect to theclock received from the clock generating unit 1101 and transmits thedetermined timing information from the clock signal interface 110. Theclock signal is split as much as the number of disposed OLTs. This splitpoint may exist inside the interface unit 1110, or may be implemented asan external connector or splitting circuit (this is merely a slightdifference in mounting, and a variation may also be made with respect toFIG. 8).

FIG. 12 is a systemic construction view when a feedback line 1200 of aclock is added to the construction of FIG. 10. The operation of disposedOLTs other than the OLT-A (1-A) is similar to that of the system shownin FIG. 10.

FIG. 13 is a block diagram of the OLT-A (1-A) of the system shown inFIG. 12. The function of the synchronization processing unit 1100 isequal to that shown in FIG. 8. That is, the clock signal generated bythe clock generating unit 1101 is transmitted from the interface 1110for clock signal to nc OLTs. Here, the clock is transferred through thelines 201-1 to 201-nc, each of which connects each OLT to thesynchronization processing unit 1100. Meanwhile, the clocks transmittedin the PON section from the OLT-A (1-A), the OLT-B (1-B), and the otherOLTs are returned to the synchronization processing unit 1100 by theline 1200 for feedback at the time of having passed the WDM 120. Thefeedback signal entered from the line 1200 shown in FIG. 12 includesplural downstream wavelengths, and therefore, the feedback signal issplit for each and every wavelength by the WDM 1350 and then O/Econverted by the clock extracting units 1320-1 to 1320-nw, so thatserial data are extracted. The clock signal extracted from the serialdata is transmitted from the clock extracting unit 1320 to a clockcomparing unit 1341. The clock comparing unit 1341 detects a differencefrom the reference clock and a clock correcting unit 1342 determines theamount of correction for each and every clock line 1310-1 to 1310-nc.The amount of correction (or corrected value) obtained herein isnotified from the clock correcting unit 1342 to each interface 1110-1 to1110-nc, and each interface outputs the corrected clock to each clockline 210. Providing the synchronization control unit 200 inside theOLT-A enables the entire PON system to be made compact compared to acase where the synchronization control unit 200 is positioned outsidethe OLT-A. This allows the size of the circuit for controlling the PONsystem to be reduced, thus making it possible to lower costs. Also,integration of the control function prevents inconsistent efficiency inthe transmission process or external noises that may be caused uponcommunication with external devices, thus making it possible to improveaccuracy of the control. In particular, this effect may be anticipatedin a case where this function is mounted in the OLT which operates atthe maximal bit rate among the disposed PONs. Further, a basicconstruction of the clock corrected value database is as shown in FIG.9.

Clock synchronization between OLTs is a process required to be used fortransmission of the downstream signals from the OLT, a case where DBAresults are collected in the individual OLT, or a case of collectinginformation generated as a frame for downstream transmission for finaltransmission control. The latter digital signal process may be adjustedat a bit buffer (receiving data buffer for clock timing adjustment)provided in the PON-IF of the OLT, and may be implemented through a busor communication cable that connects between devices without usingoptical signals.

It is important where a wavelength for downstream signals is shared bydisposed PONs during clock synchronization for downstream signals. Inthis case, the transmission clocks for the overall OLTs need to beunified to effectively multiplex the downstream frame from each OLT soas to effectively utilize the downstream band and to allow the ONU 2 toexactly receive information prior to each ONU 2.

Meanwhile, when the wavelengths for the upstream signals to be used byplural disposed PONs is equal to each other, and the downstreamwavelengths are different from each other for each PON, synchronizationof the downstream signal is not needed over the optical fiber. This iswhy it is important herein that overlapping of signals or difference inclock when the uplink signals are multiplexed do not occur, that is,OLTs receive the upstream signals. Clocks returned to the OLTs areconsidered to be varied from a fact that branch line optical fibers fromthe splitter to the ONU2 respectively have different installationconditions as well as environments such as expansion and/or contractionof optical fibers or operation temperature of the ONUs (this isassociated with stability of wavelengths of the laser, and therefore,optical transmission characteristics may vary as wavelengths arechanged, which in turn affects the clocks). It is impossible tocompletely remove it from the OLT side. Accordingly, it is needed forthis case that the OLT 1 should refer to the receiving clock from theONU 2 to vary the transmitting clock of the downstream signal withrespect to the ONU 2. Here, it is sufficient to refer to the result ofphase confirmation of the upstream frame under ranging process oroperation to obtain the clock information of the upstream signal. Sincethe phase information extracts the transmission clock timing on aper-bit basis with respect to the timing expected by the OLT, theinformation may be used not only for EqD correction for ONU 2 but alsofor minute adjustment of the upstream signal clock using the downstreamframe transmission at the OLT side.

FIG. 56 depicts the above-mentioned operation. FIG. 56 shows an order ofa transmission/receipt timing correction process between the OLT 1 andthe ONU 2 in which the upstream transmission timing has been varied byexpansion and contraction of the branch fiber. If the change of phase isdetected by phase confirmation upon receipt 5602 with respect to anupstream signal, the OLT 1 determines whether the phase is within anacceptable range or not. Here, “acceptable range” is not based on aregulation in the conventional PON (8 bits in 1.2 Gbps, or the like),but based on the conditions such as setup of DBA control period inoperating the disposed systems or avoidance of signal overlappingbetween PON interfaces. If necessary, change in EqD setup is transferredto the ONU 2. Also, clock phase is extracted from the upstream frame5601 that is received at the same time. In a case where the clock of theupstream signal is shifted with respect to the other upstream signalclocks over the optical fiber, clock phase correction 5605 is conductedin addition to EqD correction 5603.

Of course, it is needed to receive the upstream frame in compliance withthe timing of DBA processing between OLTs. It is required to make anoperation while performing correction in clock phrase or freight arrivaltiming within any constant acceptable range to meet this condition. Theminute adjustment described herein is a necessary function to facilitatethis operation. As will be described next, there may be a possibilitythat all of the ONU 2 s under the control of the corresponding OLT areneeded to adjust EqD and clock phase in a case wheretransmission/receipt timing has been varied due to difficulty in termsof DBA control. As a result, there could be considered a case where theEqD needs to be adjusted with respect to all of the disposed systems.

Also, clock synchronization for downstream signal transmission isnecessary in a case where the entire downstream communications share thesame wavelength in all of the PONs and the entire upstreamcommunications share the same wavelengths, respectively. It is common toprepare a guard time with respect to the upstream signals because theupstream signals become burst signals that serve to achievesynchronization upon receipt for each frame. Clock phase adjustment isconducted for each upstream frame using the guard time. Timing when aframe is received for each frame may be understood at the OLT side.Simultaneously, this may be responded by preparing a memory that retainsphase information of the clock shifting the upstream frame from eachONU. For this case, it is difficult for the branch fiber to unify theupstream signal clocks from each of the different ONUs, and it is usefulto reduce the guard time as much as possible to be capable ofeffectively utilizing the band.

FIG. 57 is a flowchart illustrating a process in the OLT in a case wherethe upstream, downstream, and all of the PONs have a common wavelength.Upon receipt of the upstream frame, if the receiving timing and clockphase are detected 5701, an error is detected from expected value of thetiming and phase together with ONU identifier included in the frame5702. The processes 5703 and 5704 for EqD correction are omitted herebecause their characteristics are the same as those in the existingoperations. In a case where the clock phase is shifted, ONU identifierfirstly obtained from the upstream frame is combined, and database forreceiving clock phase is referred to, so that the receiving clockinformation of the corresponding ONU is corrected as necessary (if theshift exceeds an acceptable range) 5706.

After clock synchronization is accomplished by the above-mentionedexemplary embodiments, response timing from an individual ONU for aninstruction from an OLT is controlled so as not to overlap when a signalpasses through the optical fiber 100. The method is different dependingon the setup conditions of the DBA period, however, a case where the DBAperiod of the entire OLTs is set identically will be firstly describedas the most basic exemplary embodiment.

In a case where a bandwidth intends to be shared by plural disposedPONs, the most basic method is to make the timing by (of??) calculatingthe DBA period and the assigned amount of DBA bandwidth be identical.That is, adjustment is made between OLTs with respect to a bandwidthshared at a specific time width from a specific time with bandwidthrequests from the entire ONUs gathered. Accordingly, timing clock havingthe same period as the DBA period is generated separately from the clockfor bit phase synchronization when the reference clock is supplied, andthen supplied to each OLT. By doing so, the length of period respondingto the band instruction from the OLT and the timing of the periodboundary agree with common DBA processing period setups that are allmanaged by the OLT side (synchronization control unit 200 or 1100) withrespect to the control period of the upstream signal transmitted fromthe entire ONUs, and therefore, it becomes easy to share the bandwidthassignment information by the entire OLTs, thus leading to easiness inDBA control.

Below, the response time from the entire ONUs as a basic parameter tounify the DBA calculation timing in the entire system will be explained.The conventional single PON system has been used to determine equivalentdelay (EqD) parameters. Further, DBA results should be notified from allof the OLTs to ONUs under the control of the OLTs at a common timing(That means that DBA results in each OLT shared by the entire OLTs) andthe response signal from the ONU should be time-division multiplexinglyreceived at the OLT side in order to instruct the transmission timingwith respect to the entire ONUs. Accordingly, the entire PON systemssharing the optical fiber 100 need to have a common logical distance(which indicates the response distance (time) when represented by atemporal expression). This logical distance is measured at a time whenthe physical distance of the PON section, and delay of signal processingin the ONU and OLT are summed. In the conventional PON, processing timeafter receiving a signal from the OLT is adapted to be long in the ONUlocated near the OLT and in contrast, the processing time is adapted tobe relatively short in the ONU located far from the OLT, so thatresponse time to an instruction from the OLT becomes constant, in orderto make the transmission timing from different ONUs constant accordingto the difference in distance from each ONU to different branch linefiber 101, 102, or 103 and difference in processing time in the ONU. Thedifference in length of fibers for OLT connections 111, 112, and 113should be considered in addition to unevenness in length of glancefibers 101, 102, and 103 in the system according to the presentinvention.

There are two methods considered to implement the above process. Onemethod carries out a two-step adjustment including: adjusting thelogical distance with respect to an ONU group under the control of eachOLT; and then adjusting the difference in logical distance which isdifferent from each OLT particularly to utilize the conventional OLTfunctions. The other method is to determine the common logical distancewith respect to the entire system by intensively measuring the logicaldistance of all of the ONUs by any OLT or other special devices equippedoutside the OLTs or inside one of the OLTs. The former method mayutilize the conventional distance measuring mechanism at the individualPON system level, and therefore, development costs may be reduced. Thelatter method has an advantage in that it can be easily managed becausedatabase concentrates on one place although it needs to configure a newdistance measuring system.

FIG. 14 is a conceptual view illustrating a distance difference in acase where each PON has a different logical distance. It is assumed inFIG. 14 that ONUs belonging to the OLT-A (1-A) are gathered at thelocation closer to the OLT-A than those belonging to the OLT-B (1-B).The ONU-A (2-A-min) is an ONU which is under the control of the OLT-Aand located closest to the OLT-A (1-A), and the ONU-A (2-B-max) is anONU which is under the control of the OLT-B and located furthest awayfrom the OLT-B. FIG. 14 depicts a signal flow when a response request(for example, ranging or serial number request, or the like) wastransmitted from the OLT-A (1-A) and the OLT-B (1-B) to the ONU-A andthe ONU-B. A downlink signal transmitted from the OLT-A (1-A) at thetime 1330 arrives at the ONU (2-A-min) located nearest the OLT-A (1-A)at the time 1361 as shown in FIG. 13 in a case of output time of themessages (response request) from both OLTs are the same.

Here, the ONU-A (2-A-min) waits for a constant time until a response ismade to the OLT-A (1-A) according to the EqD notified from the OLT-A(1-A). Processing time 1311 in the ONU-A (2-A-min) is a time includingthe overall signal processing times in the ONU and the EqD notified tothe ONU. When the ONU-A (2-A-min) transmits an upstream signal accordingto the timing instructed to the OLT-A (1-A) at time 1371, the OLT-A(1-A) receives the uplink signal at time 1340. Detailed descriptions onparameter definition regarding the upstream signal, including the EqD,is assumed in the exemplary embodiment to follow the recommendation ofG. 984.3 (non-patent document). While the ONU-A (2-A-min) waits, thesignal from the OLT-A (1-A) reaches the ONU-A (2-A-max), and the ONU-A(2-A-max) transmits the upstream signal to the OLT-A (1-A) at time 1372after the processing time 1312 including the EqD notified from the OLT-A(1-A). In a case where the OLT-A (1-A) performs transmissioninstructions to the whole ONU-As at the same time, the time 1340 whenthe OLT-A (1-A) receives the signal is the same among the ONU-As.

This is true for the OLT-B (1-B). The signal transmitted from the OLT-B(1-B) at the transmission time 1330 is received at times 1381 and 1382with respect to the ONU-B (2-B-min) and the ONU-B (2-B-max). The ONU-B(2-B-min) and the ONU-B (2-B-max) transmit a response message to theOLT-B at times 1391 and 1392 after the stand-by times 1321 and 1322,respectively. The OLT-B simultaneously receives the signal from bothONU-Bs at time 1350.

In the conventional PON system, setup of processing time in the ONUs isgiven as EqD, and the ONU-A (2-A-min) is set to have a larger value by avalue corresponding to the difference between the processing times 1311and 1312 in the ONU than the ONU-A (2-A-max). In the ONU-B, likewise,the EqD notified from the OLT-B to the ONU-B (2-B-min) uses a largervalue than the EqD of the ONU-B (2-B-max) by a value corresponding tothe difference between the processing times 1321 and 1322 in the ONU.

In the state shown in FIG. 14, there is a case where signals areoverlapped according to the logical distance up to the ONU in a casewhere plural OLTs receive upstream signals from the ONUs under theircontrol. This logical distance needs to be adjusted herein between theOLTs (between disposed PON systems).

FIG. 15 depicts a transmission timing of a PON upstream signal when thelogical distance is adjusted with respect to the state shown in FIG. 14.Logical distance is set therein with respect to the entire PONs based onthe ONU located furthest from the OLT. Because the ONU-B (2-B-max) islocated further than the ONU-A (2-A-max) in the case shown in FIG. 15,the setup value of the OLT-A is calculated to get the suitable value tounify the logical distance with the logical distance set by the OLT-B.

The EqD setup of the ONU-B from the OLT-B has the same order as that ofthe EqD setup of the conventional ONU in case of FIG. 14. With respectto the ONU-A, the EqD value set to each ONU-A is corrected taking intoconsideration the difference between the logical distance set by theOLT-A and the logical distance set by the OLT-B, that is, the differencebetween the communication time A1301 and the communication time B1320.Because the relative difference in distance between the ONU-As is givenby the OLT-A as shown in FIG. 14, it is preferable to add a constantvalue that corresponds to ΔD1303 to the entire ONU-As. By doing so,processing time in the ONU-A (2-A-min) becomes processing time 1411obtained by adding processing time 1311 in the ONU to ΔD1303. Similarly,processing time in the ONU-A (2-A-max) becomes processing time 1412. Asa consequence, a response from all of the ONUs arrives at all of theOLTs at time 1350 at the same time in a case where the entire OLTs issuetransmission instruction to the entire ONUs at transmission time 1330 inthis system.

FIG. 16 depicts a method of supporting a band allocation situationbetween the OLTs by providing a distance control unit 1500 outside thesystem shown in FIG. 1. The distance control unit 1500 is providedbehind the OLT, and each OLT is connected to lines for clock supplies1501-A, 1501-B, 1502, 1503, and 1510 through the distance control unit1500. The distance control unit 1500 includes a logical distancemanagement table 1800 (FIG. 19A). On the other hand, the OLT includes anEqD correction information DB 1601 (refer to FIG. 17) for retainingcorrected values of distance information. A distance informationprocessing unit 670 in the distance control unit 1500 (FIG. 18) refersto the logical distance management table (delay DB) to produce thedifference in the delay amounts between the OLTs. The amount ofcorrection for logical distance adjustment between PON systems isnotified to each OLT based on the amount of delay that may have beenobtained.

FIG. 17 depicts a block construction of the OLT-A (1-A) in the systemshown in FIG. 15. The OLT-A (1-A) includes an interface with an opticalfiber 111-A on the PON section side (ANI side). A WDM 660 plays a roleas the interface herein. The WDM 660 is used to split wavelengths of anupstream signal and a downstream signal, and provided separately fromthe WDM 120 for connecting between the OLTs as shown in FIG. 16.Ethernet (registered trademark), 10 Gbps Ethernet (registeredtrademark), and a TDM interface whose representative example includes T1and E1 may be used as the interface 610 on the carrier network side (SNIside) of the OLT-A 610-1 to 610-n.

Process of downstream signals from the OLT-A (1-A) is as follows, forexample, in the case of using Ethernet (registered trademark) as theinterface. To begin with, when a signal is transmitted to the interfaces610-1 to 610-n, the signal is subjected to a synchronization process inthe interface 610 to terminate the protocol. The receiving processingunit 621 determines the destination in the PON section of the downstreamframe based on the head information of received downstream frame. Thatis, it is determined whether the destination is located before aspecific ONU or plural ONUs, or information that should be terminated bythe OLT (that should be transferred to a CPU provided in a PON controlunit of the OLT). Also, the receipt processing unit 621 performs aheader process such as conversion, addition, and deletion of addressinformation, as necessary, in response to the destination (and source)information or type of received data. Here, the destination (and source)information includes logical path information such as VLAN tag or MPLSlabel as well as MAC and IP beginning route information. The informationto be transferred to the CPU is transmitted to the PON control unit 600and the data to be sent to the ONU is transmitted to the downstreamframe generating unit 622. The downstream frame generating unit 622makes frames for downstream transmission based on the frame receivedfrom the receipt processing unit 621 and the information from the PONcontrol unit 600. Here, the information from the PON control unit 600includes DBA information notified to the ONU or control information (forexample, PLOAM messages and the line in G. 984.3) for performing controland management of the PON section. The control information is generallyinserted into the header of a downstream frame, however, may also beinserted into the payload of the frame in a case where the ONU may berecognized for a specific use such as vendor-specific messages (theframe format is not limited in the present invention). The transmissionprocessing unit 623 buffers the frames generated by the downlink framegenerating unit 622, and reads it according to the priority of the frameinformation, the state of the destination ONU, and processingperformance, and the E/O converting unit 631 converts it into a opticalsignal and transmits it according to transmission clocks.

The following operation is performed with respect to upstream signals.When an O/E conversion unit 632 receives an upstream wavelength signalsplit by the WDM 660, serial data is reproduced based on the signal. Thetransmission clock of the upstream signal is extracted based on thereproduced signal. The received data is serial/parallel convertedaccording to the reproduced clock and then frame synchronization for theupstream frame is carried out. At this time, the reproduced clock isreceived at a timing instructed upon transmission of the downstreamframe with respect to the ONU prior to receipt of the upstream frame bythe OLT-A (1-A). The instructed timing is retained in the DBAinformation A included in the PON control unit 600 of the OLT-A (1-A)until the receipt of the upstream frame, and it is identified whetherthe upstream frame is received at a right timing upon receipt thereof.This is conducted by comparing and referring to the received clockinformation and the DBA information 602 with respect to the receivingclock identifying unit 652. At this time, if the timing is shifted fromexpected one, the EqD information A 601 retained in the PON sectioncontrol unit 600 is updated, and the EqD information before the ONU isinserted into the header of the downstream frame to correct the logicaldistance. The receipt processing unit 641 identifies whether it is datathat should be transmitted to the SNI or terminated at the PON controlunit 600 based on the header information of the received frame similarlyto the process of the downstream frame. In addition, the receiptprocessing unit 641 performs addition, conversion, and deletion ofdestination information according to the setup of path information basedon the header information of the received frame. The upstream framegenerating unit 642 generates the header and payload of a frametransmitted from the PON control unit 600 and the receipt processingunit 641 to the SNI and transmits the generated header and payload tothe transmission processing unit 643. The upstream frame generating unit642 buffers the generated frame, reads it according to priority of frameinformation, state of destination ONU, and treatment performance, andtransmits data through the SNI-sided interface 610.

The interface 1610 connected with the distance control unit 1500 is aninterface for receiving EqD correction information that is generated andsupplied by the distance control unit 1500. For example, Ethernet(registered trademark) or other interfaces may be used as the interface.Of course, an independent interface (i.e. protocol) may be used forcooperation between devices. The EqD correction amount information istransferred to the PON control unit 600 via a transmission/receiptprocessing unit 1620 and stored in the EqD correction informationdatabase 1601. Here, EqD values for the entire system are generated fromthe EqD information A 601 and the EqD correction information A 1601 withrespect to each ONU. The EqD generated herein is stored in the header ofthe downstream transmission frame and notified to each ONU.

Also, the EqD values are referred to by the clock adjustment unit 651.The clock adjusting unit 651 has a function of managing a difference inclock phase between the downlink signal and the uplink signal.

Thereafter, receiving timing of the upstream signal is obtained or EqDis readjusted in case of the difference in clock on the basis of the EqDset herein.

FIG. 18 depicts a block construction of the distance control unit 1500in the system shown in FIG. 16. The distance control unit 1500 includesa communication interface with Ethernet (registered trademark), 10 GbpsEthernet (registered trademark), or a TDM interface whose representativeexample is T1 and E1, or an independent interface on the OLT side(Network Node Interface or Network Network Interface (NNI) side).

A signal process flow of the distance control unit 1500 is as follows,for example, in a case where Ethernet (registered trademark) is used asthe interface between the OLTs. To begin with, when a signal is enteredto the interfaces 1501-1 to 1501-nc, the signal is subjected to asynchronization process in the interface 1710 to terminate the protocol.The receipt processing unit 1721 identifies the OLT, which is the objectof the response time information that is stored in the frame based onthe header information of the received frame. The receipt processingunit 1721 extracts the OLT identification information or received data(delay time). Here, the OLT identification information may employlogical path information such as VLAN tag or MPLS label as well as MACand IP beginning route information. The extracted information istransferred to the delay managing unit 1750 to calculate the logicaldistance between PONs. The downstream frame generating unit 622 createsan EqD corrected value notification frame that includes a destinationOLT identifier in the header information based on the information storedin the EqD correction DB 1752 of the delay managing unit 1750. Thecorrection information is generally inserted into the payload of theframe, however, may be stored in the payload of the frame in a casewhere an ONU may be recognized for a specific purpose such as vendorspecific OAM messages.

In a case where the uplink frame receiving timing is shifted from theexpected (predetermined) value with respect to each OLT, EqD iscorrected with respect to each OLT, and simultaneously, the logicaldistance of the corrected result (logical distance up to the ONU locatedfurthest away) is notified to the distance control unit 1500. Thedistance control unit 1500 stores the received EqD information in thedelay DB 1751 and recalculates the EqD corrected value in each OLT basedon the value. The mechanism for the conventional PON may be employed asthe mechanism that corrects the EqD parameter. In a case where thetransmission timing from the ONU is greatly shifted from the set value,the logical distance needs to be reset including the values stored inthe distance control unit 1500. In a case where the amount of correctionof expected value is small, adjustment may be made by processing onlythe inside of the OLT, and at this time, using it only with theconventional function has a merit in that developing costs are reduced.In addition, the recalculation of the EqD corrected value may be made bynotifying the timing when the EqD correction is performed to theindividual OLT as well as periodically notifying the EqD informationfrom the distance control unit 1500 to each OLT and using the results.Because distance measuring information may be collected from each OLTfor each DBA period through the polling process, fine adjustment may beperformed correspondingly, thus making it possible to always monitordistance variation. For example, variation in distance information isnot useful when the EqD correction inside of the OLT cannot be conductedefficiently to support the continuity of communication, such as rapid(large) change that is difficult to follow by OLT itself occurs, and asa result, this causes the data to disappear.

FIGS. 19A and 19B depict an example of a construction of a databaseincluding information that should be retained to adjust the logicaldistance between PON systems as described in FIGS. 16 to 18.

FIGS. 19A and 19B show a table construction that is retained in each PONsystem, that is, each OLT.

A table 1800 includes EqD 1802 and the other information such as flag1803 at each ONU to correct a relative distance difference between theidentifier 1801 of the ONU under the control of each OLT and the ONU.For example, it is considered to mount a flag that represents whetherthe ONU is effective (active) or not with respect to the other flags.

FIG. 19B depicts a table to retain the amount of correction for EqD 1811when being compared with the reference PON. The logical distance may beseen with respect to refereeing all of the disposed PON systems by usingthe correction amount of EqD of the table 1810 and the EqD setup valuesof the EqD 1802 stored in the table 1800. The logical distance is arelative value for the reference value of the entire system. This valueis set as EqD for each ONU.

FIG. 20 depicts a constructional example of a table for adjusting thearrival time of the upstream frame, which is a response from ONTs toeach OLT, between the OLTs by intensively managing the response time inthe entire OLTs. The EqD corrected value management table 1900 isretained in the delay managing unit 1750 included in the distancecontrol unit 1500 shown in FIG. 18. The EqD corrected value managementtable 1900 includes the identifier 1901 of each OLT, the response delaytime notified from each OLT, that is, the logical distance 1902 fromeach OLT to the ONU located furthest from the OLT, and eachcorresponding amount of delay value to be corrected 1903. It becomespossible to set equivalent delay parameters uniform over all of thesystems sharing the optical fiber based on distance measuringinformation individually obtained from each OLT through this table. Inaddition, a method may be taken as an example of a calculating method ofdelay corrected values, which selects the one having the largest delayamong response time notifications from the OLT and sets it as areference value. As another method, it may be possible to calculate thecorrected values for each OLT on the basis of any constant value that islarger than the amount of delay actually measured.

FIG. 21 depicts a system construction when the distance control unit1500 shown in FIG. 16 is provided inside a specific OLT (OLT-A (1-A)herein). Providing the distance control unit 2000 inside the OLT-Aenables the entire PON system to be made compact compared to a casewhere the distance control unit 2000 is provided outside the OLT-A. Thisallows the size of circuit for controlling the PON system to be reduced,thus making it possible to lower costs. And, integration of the controlfunction may avoid uncertainty such as inconsistent efficiency in thetransmission process or external noises that may be caused whencommunicating with external devices, thus making it possible to haveaccurate control. In particular, this effect may be anticipated whenthis function is added to the OLT that operates at the maximum bit rateamong the disposed PONs.

In this case, the basic function of the distance control unit 2000 isequal to that when the distance control unit 2000 is positioned outsidethe OLT-A as shown in FIG. 16. The OLT-A has a function of transferringits own clock to the other OLTs like the distance control unit 1500shown in FIG. 16, in addition to a function of generating a clock by itsown frequency oscillator to communicate clock information with the ONUthrough the downstream signal.

In this case, the device construction of the other OLTs than the OLT-A(1-A) is equal to that shown in FIG. 17.

FIG. 22 depicts a block construction of the OLT-A (1-A) shown in FIG.21. In this case, the PON control unit 600 shown in FIG. 17 includes adelay DB 2101 and an EqD correction DB 2102. The PON control unit 600stores the delay information collected from an individual OLT in thedelay DB 2101 and calculates the EqD correction amount that should benotified to each OLT, including the PON control unit 600 in the OLT-A ifnecessary, based on the information. The calculated result istransmitted through the delay information communication interface 2110.The delay information notification is split as many as the number ofdisposed OLTs. The split point may be implemented by an externalconnector or split circuit even though being existent inside theinterface 2110.

This construction may perform the same operation as in the exemplaryembodiment of FIG. 16 by storing the table shown in FIGS. 19 and 20 inthe delay DB 2101 and the EqD correction DB 2102.

A method of intensively managing the delay information by the distancecontrol unit 1500 that is provided inside or outside the OLT has beendescribed so far. A method of notifying the delay information by eachOLT to others may be considered as the distance control method. Next,another example of the embodiment will be exemplified. In this case,constructions of the system and the device are identical to those shownin FIGS. 21 and 22. Hereinafter, database included in the PON controlunit of each OLT will be described.

FIGS. 23A and 23B depict an example of a construction of a databaseincluding information that should be retained to adjust the logicaldistance between PON systems as described above with reference to FIGS.21 and 22. FIGS. 23A and 23B depict a table construction that isretained in each PON system, that is, each OLT. A table 2200 includesEqD 2202 and the other information such as flag 2203 at each ONU tocorrect a relative distance difference between the identifier 2201 ofthe ONU under the control of each OLT and the ONU. For example, it isconsidered to mount a flag that represents whether the ONU is effective(active) or not with respect to the other flags.

FIG. 23B depicts a table equipped in OLTs to retain the EqD correctedvalue 2211 when being compared with the reference PON. The logicaldistance may be seen with respect to referencing all of the disposed PONsystems by using EqD corrected values of the table 2210 and the EqDsetup values of the table 2200. The logical distance is a relative valuefor the reference value of the entire system. This value is set as EqDfor each ONU. The EqD correction information 1601 retains the table asshown in FIGS. 19A and 19B.

FIGS. 24A and 24B depict a construction example of the delay DB 2101.The table 2310 retains an OLT identifier 2311 for identifying areference OLT. This enables each OLT to identify an OLT which issubjected to a comparison of arrival time in case of measuring thelogical distance from the ONU and determining EqD. The OLT-ID 2301 thatis uniquely set to the system is required for this comparison. Each OLTstores the OLT-ID 2301 set thereto therein. FIGS. 24A and 24B representan example in which the identifier of the device is registered as thefield 2312 in the table 2210. The EqD corrected value managing table2300 is equal to the table retained in the delay managing unit 1750provided in the distance control unit 1500 shown in FIG. 20. The EqDcorrected value managing table 2300 includes the identifier 2301 of eachOLT, the response delay time notified from each OLT, that is, thelogical distance 2302 from each OLT to the ONU located furthest from theOLT, and each corresponding delay corrected value 2303. In addition, theone having the largest delay is selected and used among response timenotifications from the OLT with respect to calculation of the delaycorrected value. Or, it may be possible to calculate corrected valuesfor each OLT by setting any constant value larger than the amount ofdelay measured actually measured as a reference value. In the exemplaryembodiment, an OLT measures the distance up to the ONU under its controland grasps the maximal distance in disposed group including the otherOLTs connected thereto. The additional installment of OLTs is easybecause this information has been studied autonomously. Also, the sizeof the hardware design may be suppressed such as the amount of memoryuse or circuits because the database retained in an individual OLTbecomes small compared to the intensive management.

In a case where an upstream signal wavelength is shared, transmissionpermission timing needs to be controlled with respect to the ONU so thatthe upstream signals from the entire ONUs are not overlapped over theoptical fiber 100. The response time of the overall PON sections may begrasped by the above-mentioned clock synchronization and EqD adjustmentbetween the PONs. Control of boundary of DBA period and control of DBAperiod field are necessary between the PON systems to conduct anintegrated DBA control over the entire system. An operation of theentire system is as described above with reference to FIG. 2.Hereinafter, a band control method of time-multiplexing the uplinksignals between the PON systems will be described.

FIG. 25 depicts a state of having time-divided upstream communicationbandwidth over a shared optical fiber between the PONs with respect tothe PON disposed system shown in FIG. 1. For ease of descriptions, twosystems including OLT-A (1-A) and OLT-B (1-B) will be described as anexample. Actually, any numbers of PONs may be disposed, and thedescriptions may be easily applied to this case, too.

In the individual PON section, there is set a response time as a systemthat is calculated by each OLT from the distance from each ONU under thecontrol of the OLT to the OLT and response processing time for a messagefrom the OLT in each ONU. This system response time is determined by asignal arrival time that comes with the distance between the OLT and theONU (optical fiber length) and information processing time required foran electrical signal process from receipt of a downstream signal totransmission of an upstream signal as its response in the ONU.Generally, the distance from the OLT to the individual ONU is notconstant but have a predetermined distribution. Accordingly, delay time(equivalent delay; EqD) which is different for each ONU is set so thatthe response timing from all of the ONUs is equal for the responseinstruction from the OLT in order to adjust the difference in distancebetween the ONUs so that the OLT may receive the correct signals fromthe ONU under its control.

Upon receiving an upstream data transmission request from the ONU underthe control at a certain timing, the OLT determines the amount oftransmission of the upstream signal for each ONU based on the requestband. If the amount of transmission for each ONU, correspondinginformation is loaded on a downstream signal to be notified to all ofthe ONUs, and the ONUs transmit the permitted amount of transmissionwith an upstream signal. This series of processes are a PON-specificcontrol method that is called “DBA”.

The upper half of FIG. 25 depicts timing when downstream frames aretransmitted to ONUs that belong to each OLT from OLT-A (1-A) and OLT-B(1-B). The temporal axis is shown from right to left of the drawing,wherein a more rapid process is conducted at the right side. Framesshown in the right side are transmitted more quickly. Scales marked inparallel with the temporal axis refer to a period of 125 microseconds,for example, in case of G-PON at the boundary of period for timingcontrol that performs periodic frame transmission. It is generallydifficult to control DBA on a per-125 microseconds basis (hereinafter,per-basic period frame) in view of CPU performance, and the DBA controlis performed with plural basic period frames bundled. In FIG. 25, time330-A from time 360-A to time 361-A and time 330-B from time 360-B totime 361-B, respectively, represent a DBA control period for OLT-A (1-A)and OLT-B (1-B). It is assumed herein that the 125 microsecond periodtiming and DBA period timing are supplied to both OLTs in addition tothe clock synchronization and EqD adjustment, so that the DBA period hasbeen synchronized. Accordingly, times 360-A and 360-B come to have thesame timing, and the DBA periods 330-A and 330-B come to have the sameperiod.

Under conditions where a response time of the ONU is uniform over all ofthe system, upstream signals from the ONU may be time-divisionmultiplexed by adjusting the DBA instruction timing on the OLT side. TheOLT-A (1-A) and the OLT-B (1-B) transmit downstream frames at differentwavelengths from each other. Frame 310-1 that is composed of header 310h-1 and payload 310 p-1 to frame 310-Nda, and frame 320-1 to frame320-Ndb are included in one DBA period (period for band notificationfrom OLT to ONU).

The band notification for ONU that is managed by each OLT is transmittedas header information of the downstream frame. In FIG. 25, as anexample, the band notification 300-A from the OLT-A and the bandnotification 300-B from the OLT-B to the ONU are allocated to the frontpart of the DBA period 330-A (that is, 330-B) and the rear part of theDBA period 330-B, respectively. In this case, it is represented that DBAcalculation period is unified in the entire system. It is, however,bandwidth allocation timing is different for each system.

Lower half of FIG. 25 represents a time when the band instructionreaches the ONU and a time when upstream signals are received to eachOLT when seen from the temporal axis in the OLT-A (1-A) and the OLT-B(1-B). The frame having the band notification 300-A from the OLT-A (1-A)as the header information arrives at the ONU-A from the time 370-A tothe time 371-A. The ONU-A transmits uplink frames to the OLT-A after aconstant stand-by time according to the logical distance notified fromthe OLT-A. The processing time 350-A includes the transmission delaytime of the uplink signal and processing time in the ONU. The responsetime seen from the OLT-A is a time from time 360-A to time 390-A. Thisis applied for the OLT-B; the response time is from the time 360-B to390-B.

By this process, the upstream signals in the DBA period 401 that isinstructed at the DBA period 330-A (330-B) may be normally receivedwithout overlap occurring. At the communication time 340-A used by theOLT-A (1-A), the standby time 410-B may be allocated to the OLT-B (1-B),and at the communication time 340-B, the standby time 410-A may beallocated to the OLT-A (1-A).

FIGS. 26A and 26B depict a construction example of a bandwidthallocation table retained by the OLT. The bandwidth allocationinformation calculated in the PON control unit 600 based on a bandwidthrequest from the ONU under the control of the OLT and an upstreambandwidth allocation state of the other PONs is stored in the tableshown in FIGS. 26A and 26B. The OLT reads the entry of the table in theupstream bandwidth instruction timing that may have been allocated tothe OLT, loads it on the head information of the downstream frame, andtransmits it to the ONU under its control. FIG. 26A depicts aconstruction example of a table in a case where a band is allocated bythe OLT in such a manner to follow existing recommendations with respectto a band control ID 2501 (generally, association with ONU ID is madeupon setup/start of ONU) that is managed by the OLT within an upstreamcommunication time allocated to the OLT.

The allocation band 2502 is yielded based on the amount of allocation ofupstream transmission time for each OLT that may be obtainable on thebasis of (1) a communication request from the ONU under the control ofOLT and (2) mutual communication between the DBA control units 2600(refer to FIG. 27) or OLTs. The other information 2503 is considered tobe used to represent, for example, entry management (valid or invalid)of the bandwidth control ID 2501 according to allocation condition inthe DBA period (whether completed or non-allocated) or service usecondition. The present invention is not particularly restricted to acertain method of using the flag. FIG. 26B depicts a table constructionwhen the OLT determines an upstream communication bandwidth (time slot)allocated thereto and allocates a valid band to the ONU under itscontrol only within the range. In the instruction within thecommunication time 2505 regarding the bandwidth control ID 2501, aneffective value is set to the allocation bandwidth 2502 so thattransmission permission is issued to the corresponding ONU. A zerobandwidth is allocated to the ONU under the control of the OLT to stopthe communication in the time when the other ONUs should conductcommunication, that is, stand-by time 2506. Data from the bandwidthallocation amount 2502 is transmitted to the ONU from the bandallocation location 2504 in response to the instruction from the OLTbased on the table. In addition, although not shown herein, it is alsopossible to specify the allocation start time (location) 2504 and theallocation end time (location) besides the combination of allocationstart time 2504 and allocation bandwidth 2502 with respect to the timinginstruction of the upstream communication. In the latter case, theallocation end time may be stored instead of the field of the allocationbandwidth 2502.

The construction itself of the band allocation table is equal to that ofthe conventional PON system, except that applicable upstream bandwidthassumed for DBA calculation is varied with each DBA period in thepresent invention, whereas applicable upstream bandwidth alwayscorresponds to all of the DBA period in the prior art. There areconsidered plural methods as a mounting method. For example, there maybe provided, for example, a method of performing table access onlyduring a necessary time by reading a table and controlling the timing,and a method of performing table access similarly to the prior art inthe DBA period by placing a zero bandwidth with respect to an entry thatis accessed other than the bandwidth allocation timing. For example, thetable shown in FIG. 26A and the table shown in FIG. 26B may be appliedto the former method and the latter method, respectively. In the former,the size of hardware may be small since it is to prepare memory space asmuch as the number of entries of the band control ID 2501 that issubjected to its band control. Also, the operation mechanism of DBAitself may be equal to that of the conventional system, and therefore,this has an effect of being capable of reducing development costs. Thelatter method can realize desired operations by keeping the tableextraction period constant to control DBA according to the conventionalmechanism. Even though allocation upstream communication time is changedfor each OLT, it is sufficient to select a corresponding bandwidthcontrol ID 2501 to set permissible communication data amount fornecessary entry, and it is not necessary to rewrite the registrationorder of the entry itself.

Accordingly, system designing is relatively easy.

FIG. 27 depicts a method of managing a bandwidth allocation conditionbetween OLTs by providing a DBA control unit outside the system shown inFIG. 1. The DBA control unit 2600 is provided behind the OLT, andconnected to each OLT through each of lines for DBA control 2601-A,2601-B, 2602, 2603, and 2610. The DBA control unit 2600 includes abandwidth allocation management DB 2752 (FIG. 28). The OLT includes abandwidth information DB 602 (refer to FIGS. 26A and 26B). The DBAcontrol unit 2600 refers to the bandwidth allocation management DB 2752and notifies bandwidth allocation information to each OLT. In addition,the lines for DBA control may be shared with the lines 1501, 1502, 1503,and 1510 for notifying logical distance control information.

FIG. 28 depicts a block construction of the DBA control unit 2600 shownin FIG. 27. The DBA control unit 2600 includes a communication interfacesuch as Ethernet (registered trademark), 10 Gbps Ethernet (registeredtrademark), and a TDM interface whose representative example includes T1and E1, or an independent interface on the OLT side (NNI side).

A signal process order of the DBA control unit 2600 is equal to thefollowing flow in a case where Ethernet (registered trademark) is usedas the interface. To begin with, when a signal is entered to interfaces2601-1 to 2601-nc, the signal is subjected to a synchronization processin the interface 2710 to terminate the protocol. The receipt processingunit 2721 identifies the OLT which is an object of bandwidth requestinformation stored in the frame based on the header information of thereceived frame. The receipt processing unit 2721 extracts OLTidentification information or receiving data (bandwidth request). Here,information on MAC or IP beginning route as well as logical pathinformation such as VLAN tag or MPLS label may be used as the OLTidentification information. The extracted information for adjustingupstream bands that may be used at the individual PON is transferred tothe bandwidth management unit 2750. The downstream frame generating unit622 makes a usable bandwidth notification frame including destinationOLT identifier in the header information based on information stored inthe bandwidth allocation management DB 2752 of the bandwidth managingunit 2750. The bandwidth information is generally inserted into thepayload of the frame, however, may be stored in the payload of the framein a case where an ONU may be recognized with respect to a specific usesuch as vendor-specific message and the like. Here, the bandwidthinformation includes transmission start timing, or a combination oftransmission start timing and transmission end timing.

The construction of the functional blocks for OLTs is identical to thatshown in FIG. 17 in the exemplary embodiment. In FIG. 17, the interfacefor notifying bandwidth information and the transmission/receiptprocessing unit may be replaced with the logical distance informationnotification interface and the transmission/receipt processing unit,respectively. These interfaces may include both functions or separatelyinclude each of both functions. Such a difference in mounting does nothave an effect on the point of the present invention.

FIGS. 29A and 29B depict a construction example of a table forcalculating a use bandwidth of each OLT in the DBA control unit 2600.The band request management table 2800 is retained in the bandwidthmanagement unit 2750 included in the DBA control unit 2600 shown in FIG.28. The bandwidth request management table 2800 includes an identifier2801 of each OLT, a request bandwidth 2802 notified from each OLT, apriority 2803 for each OLT, and a flag 2804. The priority 2803 and theflag 2804 are optional. The priority 2803 may be used in a case wherethe sum of the request bandwidth 2802 from all of the OLTs exceed anacceptable band of a line. Use of upstream bandwidth is sequentiallypermitted from the OLT having high priority on the basis of the priorityset for each OLT. Also, it is possible to distribute bandwidth permittedat the ratio of corresponding to the priority. The flag 2804 may be usedto identify the state of each entry such as whether or not allocation ismade on each OLT, or whether bandwidth allocation has been complete fora certain OLT in a case where band for the OLT is allocated with thebandwidth separated into a few periods. Also, it may be possible toseparate the type into a fixed rate communication having high priorityand a best effort communication having low priority from each OLT toreceive a standby request in order to effectively use bands. An exampleof a table construction for this case is shown in FIG. 29B. It ispossible to firstly secure bands for fixed rate communication withrespect to bandwidth calculation in the DBA control unit including theinformation for each type 2805.

FIGS. 30A and 30B depict a construction example of a bandwidthallocation table for managing use bandwidth of each OLT in the DBAcontrol unit. The band allocation management table 2900 is retained inthe bandwidth managing unit 2750 included in the DBA control unit 2600shown in FIG. 28. The bandwidth allocation management table 2900includes an identifier 2901 for each OLT, and a request band 2902notified from each OLT. Also, it may be possible to separate the typeinto a fixed rate communication having high priority and a best effortcommunication having low priority from each OLT to receive a standbyrequest in order to effectively use bandwidth. An example of a tableconstruction for this case is shown in FIG. 30B. It is possible tofirstly allocate bandwidth for fixed rate communication to each OLTincluding the information for each type 2903. FIGS. 29A and 29B depict aconstruction of a table for storing information notified from the OLT,and FIGS. 30A and 30B depict a table for retaining allocation ofbandwidth that is produced by the DBA control unit 2600 based on theinformation shown in FIGS. 29A and 29B. With respect to the OLTreceiving a bandwidth request, the priority 2803 or the type 2805 isconsidered and then the allocation band is determined. Allocation ofbandwidth is set as shown in FIG. 30A with respect to each OLT. In acase where the type 2805 is given, bandwidth for each type are retainedas shown in FIG. 30B. Information on the table shown in FIGS. 30A and30B is notified to each OLT together with yielded conditions. In a casewhere the conditions have been yielded for each type, bandwidthpermitted for each type are notified. It is inevitable to retain theinformation to realize the DBA control. Actually, this table is preparedin two pieces, so that a bandwidth request for the next period from theOLT is written on the bandwidth request storing table (FIGS. 29A and29B) while DBA calculation is conducted based on the bandwidth requeststored in one table (FIGS. 29A and 29B). Respective of the bandwidthnotification, also, while permitted bands are calculated by a DBAfunction and written on a part of a bandwidth allocation informationtable (FIGS. 30A and 30B) prepared in two pieces, allocation informationcompleted to be written on the other part of the bandwidth allocationinformation is read out and notified to each OLT.

In addition, it is preferable to explicitly provide a bandwidth whichmay use each OLT, that is, communication variation range, to informationto be notified to the OLT. In FIGS. 30A and 30B, the allocation starttime 2904, the allocation bandwidth 2902, and the time slot that eachOLT may use are determined. It may also be possible to determine thetime slot by providing the allocation start time 2904 and the allocationend time other than the combination described above, which is not shown.It is not necessary to explicitly instruct the time slot from the DBAcontrol unit 2600, which is not to use a table that does not include theallocation start time 2904. At this time, it is sufficient tosequentially allocate the upstream bandwidth from the head of theapplicable bandwidth based on a predetermined order such as priority ofOLT or bandwidth request arrival order from the OLT.

Even though it has been described to assume two-step process in which arequest is notified to the DBA control unit after information has beencollected from the ONU under the control of OLT, it may be possible todirectly notify a bandwidth request from the ONU to the DBA control unitso that the DBA control unit directly determines band allocation for theentire ONUs. A construction example of the bandwidth allocationmanagement table in the DBA control unit 2600 for this case is shown inFIG. 31. The basic construction of the table is identical to that shownin FIG. 26A, except that the corresponding OLT identifier 3001 is storedin the table as a destination notifying the bandwidth allocationinformation to an individual ONU. To construct this table, each OLTcollects the bandwidth request from the ONU under its control andtransmits the information to the DBA control unit 2600. The DBA controlunit 2600 collects band requests from all of the ONUs sharing a trunkline optical fiber of the ODN, produces a permitted bandwidth for eachONU, and stores the result to the bandwidth allocation (management)table. Because DBA process is distributed for each OLT in the former,the size of memory necessary for each device (OLT) is reduced, thusleading to an effect in terms of device size or necessary costs. In thedistributed processing, also, the function required for each device ismostly based on the existing specifications, thus providing a merit thathas little effect on the existing device. In the latter, the DBAinformation is integrally managed and this makes it difficult to createa shift of operation between PON Interfaces (PONIFs) in the system.Since the DBA control unit 2600 may determine the former information,the guard time may be small which offsets instability of operations thatshould be set after taking into consideration of the operationalaccuracy in an individual OLT when being compared with a case ofdistribution management. Accordingly, it is possible to use the upstreambandwidth more effectively.

FIG. 32 is a flowchart illustrating a processing order of a DBA controlmethod in the system shown in FIG. 27. A basic flow of the DBAprocessing will be described herein using the DBA control unit 2600. Foreasiness of descriptions, the descriptions will be made assuming thateach of OLT 1 and ONU 2 represents plural devices. A frame #i (3201)from the OLT 1 to the ONU 2 includes bandwidth permission informationproduced based on a bandwidth request included in an upstream frame thatis received from the ONU 2 prior to the frame #i (3201). By the upstreamframe #i (3202), the ONU requests an upstream transmission bandwidth atthe next period with respect to the OLT 1 on the basis of thetransmission standby information amount that is accumulated in itsupstream transmission queue. The OLT condenses the request from the ONUunder its control and notifies the total band required to the PONmanaging the OLT to the DBA control unit 2600 (3207). The DBA controlunit 2600 arranges the bandwidth requests from the entire OLTs in thetable shown in FIGS. 29A and 29B, calculates the bandwidth allocationamount based on the information, and stores the result in the tableshown in FIGS. 30A and 30B. The bandwidth calculated by the DBA controlunit 2600 is notified from the frame 3208 to each OLT. The OLTcalculates the bandwidth allocation amount for the ONU under its controlbased on the information from the DBA control unit 2600 and stores it inthe bandwidth allocation information table therein. Thereafter,permitted band is transmitted to the ONU based on the table.

In FIG. 32, control timing of bandwidth information is differentaccording to unevenness of distance up to each ONU that is included inthe system and setup of DBA period for each PON. For example, assumingthat the DBA period is set to be equal for all of the OLTs and thedistance to all of the ONUs is roughly the same as a basic case, when anindividual OLT in a period collects the requests from the ONUs under itscontrol, the OLT condenses them in the DBA control unit 2600 at the nextperiod. At the next period thereto, bandwidth permission amount iscalculated for each OLT with respect to the DBA control unit 2600, andat the next period, calculation result is notified to each OLT. Each OLTcalculates the allocation amount to the ONU under its control in abandwidth first given thereto, and at still the next period, downstreamframe including band instruction before the ONU are transmitted.

In a case where band allocation is intensively conducted on the DBAcontrol unit 2600, it is possible to further shorten time untilbandwidth allocation is determined after a band request is transmittedfrom the ONU. As soon as the OLT 1 receives a request from the ONU 2,the OLT 1 notifies it to the DBA control unit 2600. The DBA control unit2600 calculates bandwidth allocation for the entire ONUs at the stage(at the next period) roughly provided with the request from the ONU. Atthe next period, calculation result is notified to the ONU.

FIG. 33 depicts a system construction when the DBA control unit 2600shown in FIG. 27 may have been provided in a certain OLT (OLT-A (1-A)herein). In this case, basic functions of the DBA control unit 2600 areequal to those when it is provided outside the OLT as shown in FIG. 27.The OLT-A (1-A) generates a clock by its frequency transmitter, and hasa function of transferring the clock to the other OLTs similarly to theDBA control unit 2600 shown in FIG. 27, in addition to a function ofcommunication clock information with the ONU through a downstreamsignal. In this case, device construction of the other OLTs than theOLT-A (1-A) is identical to that shown in FIG. 17.

FIG. 34 depicts a block construction of the OLT-A (1-A) shown in FIG.33. In this case, the PON control unit 600 shown in FIG. 17 includes abandwidth request DB 2101 and a bandwidth allocation DB 2102. The PONcontrol unit 600 retains band request information collected from eachOLT in the bandwidth request DB 2101, and calculates the bandwidthallocation amount that should be notified to each OLT based on theinformation. The calculation result is transmitted through the bandwidthinformation communication interface 3310. Delay information notificationis split as much as the number of disposed OLTs. The split point may beexistent inside the interface 3310, or implemented as an externalconnector or a splitting circuit.

In this configuration, tables stored in the bandwidth request DB and thebandwidth allocation DB, respectively, are identical to the tablesretained in the DBA control unit of the system. Further, the order ofbandwidth control is identical to the order shown in FIG. 32 except thatthe OLT-A (1-A) is replaced with the DBA control unit. Providing thesynchronization control unit 200 inside the OLT-A enables the entire PONsystem to be made compact compared to a case where the synchronizationcontrol unit 200 is positioned outside the OLT-A. This allows the sizeof circuit for controlling the PON system to be reduced, thus making itpossible to reduce costs. Further, integrating control functionsprevents inconsistent efficiency of the transmission process or externalnoises caused when performing communication with external devices, thusmaking it possible to improve accuracy of control. In particular, thiseffect may be anticipated in a case where this function is mounted inthe OLT that operates at the maximum bit rate.

With respect to DBA control, a case is considered where OLTs notify bandallocation information to each other. In this case, the construction ofsystem and device is identical to that described above with reference toFIGS. 21 and 22. Database retained in the PON control unit of each OLTwill be described hereinafter.

FIGS. 35A and 35B depict an example of a database construction forautonomously sharing a band allocation condition between PON systems.FIGS. 35A and 35B show a table construction that is retained in each PONsystem, which is in each OLT. The basic functions of the table 3400 areidentical to those shown in FIG. 30A. The table 3400 includes a summedvalue 3402 of bandwidth allocated to the ONUs under the control of theOLT, the start location 3403 of the bandwidth, priority 3404 between theOLTs in the bandwidth allocation process, and the maximum value 3405that may be allocated to each OLT. The priority 3404 is required toperform an autonomous process, and this may be allocated upon setup ofthe system and frequently updated according to previous band usesituation. If bandwidth allocation is conducted independently on the ONUunder the control of the OLT having each optical fiber, there could bepossibility that upstream signals are overlapped, thus failing to makecommunications. A DBA process is performed therein, and the priority foreach OLT is referred to, which is set in the table. If bandwidthallocation process is not ended for the OLT having higher priority thanthe OLT, bandwidth allocation stands by without performing. If there areonly OLTs having lower priority than the OLT, the bandwidth iscalculated which is used by the ONU under the control of the OLT, andthe calculation result is registered in the table shown in FIGS. 35A and35B and simultaneously notified to the other OLTs. Further, the maximumapplicable band is an option that may be set so that the bandwidth arenot monopolized by a certain OLT alone.

FIG. 35B depicts a table for grasping the bandwidth allocation order ofeach OLT for each DBA period and the band that may be remained andallocated at that time by the OLT. In a case where the autonomous bandallocation is conducted, it is needed to maintain the priority of theOLT in the device. The priority may be registered by an administratorupon start of the system, and the mutual priority may be varieddynamically according to lapse of the bandwidth allocation process(accumulated use bands or right-before band use condition). Theremaining band amount 3411 is needed to calculate the bandwidthallocation amount to the ONU. In a case where the total value of therequest bandwidth from the ONU exceeds the remaining bandwidth 3411, apossible amount of bandwidth is allocated to the ONU at the ratio ofcorresponding to the request. In addition, in a case where the maximumband 3405 is set, the same process is performed. In a case where theremaining bandwidth is smaller than the maximum applicable band 3405,the bandwidth allocation process is performed firstly for the remainingbandwidth. The bandwidth allocation process itself may be performed byeither of the conventional PON or the method according to the presentinvention, which has been described so far.

In the exemplary embodiment, the OLT manages the bandwidth allocation tothe ONU under its control and bandwidth use conditions in the disposedgroup that includes the other OLTs connected to each other. Theinformation is autonomously studied, so that additional installment ofOLTs is easy. Further, since the database retained in the individual OLTbecomes smaller when being compared with the intensive management, theuse amount of memory or size of designing hardware such as circuits maybe suppressed.

FIG. 36 depicts a state where ONUs under the control of the OLT-A (1-A)have been added to the system shown in FIG. 1. The ONU-A-N+1 (2-A-N+1)is split from a splitter 150 at the optical fiber 101-A-N+1. Because ofbeing under the control of the OLT-A (1-A), a filter 110-A-N+1 isidentical to the ONU-A-1 to ONU-A-N. If connection to the ONU-A-N+1 isdetected, the OLT-A (1-A) initiates a startup process of the ONU-A_N+1.In particular herein, a ranging process will be described as therepresentative. In a case where a control message is transferred in theenvironment when other systems coexist with respect to a certain ONUupon the startup control, the same method may be applicable, andtherefore, the description will be not limited in terms of use case.

FIG. 37 is a flowchart illustrating a ranging process in the OLT-A(1-A). If it is detected that an ONU is newly connected, registrationand connection environment setup of the ONU are initiated. Or, this isalso true for a case where the setup initiation is instructed throughmanagement software (3601). With respect to the new ONU, synchronizationof downstream signals, obtaining of frame header information, andtransmission/registration of ONU identification numbers for OLT areperformed on the ONU side.

If the ONU identifier is registered, a ranging process is conducted thatmeasures the logical distance of an optical line section fromcommunication time between the OLT and the ONU.

If a ranging request is transmitted from the OLT-A (1-A) to the new ONUfor requesting a response, however, a signal is transmitted from anotherONU to another OLT, there could be possibility of losing a signal fromthe ONU under start.

This is why the distance to the ONU newly connected is unclear yet, andtherefore, the OLT-A (1-A) may not expect having received a responsefrom the ONU at which timing after transmitting a ranging requesttherefrom.

After determining the timing of ranging (3602) and before transmitting aranging request, a communication stop is notified to the entire OLTs,each having an optical fiber 100. Subsequently, a ranging request isnotified (3604), and it is identified whether it is possible to normallyreceive a response to the ranging request or not (3605) to determine thelogical distance (3606). EqD determined herein considers only the ONUsbelonging to the OLT-A. In actually, however, the logical distance ofall the ONUs sharing the optical fiber should be considered. After EqDis determined, parameter adjustment is performed referring to the EqDinformation retained in another OLT (3607). The present inventionfeatures a process of notifying the ranging timing to the other OLTs(3603) and an EqD adjustment process 3607 in the entire system. Theprocess 3607 will be described below.

FIG. 38 is a flowchart illustrating a process from receipt of a rangingresponse to EqD determination. The EqD calculated from the rangingresponse with respect to the process 3701 is a relative value when theobject is the ONU under the control of the OLT-A. The EqD databaseregistered in the system is referred to in order to adjust the entiresystem (3702). The EqD database is retained in the PON control unit ofthe OLT. In a case where the distance to the ONU newly registered isshorter than the value set in the system as the referred result, the EqDset to the new ONU may be corrected with reference to the value set inthe system. In the meanwhile, for example, in a case where the new ONUis positioned farthest in the system, logical distance setup of theentire system needs to be varied according to the newly registered ONU.These series of processes become process 3703 and 3704.

After the logical distance in the system and the EqD to be set to theONU are all determined, the EqD setup value is notified to the ONU(3705). Further, the EqD notified to the new ONU is registered in theEqD information DB 601 included in the PON control unit 600 of the OLT.

This flowchart features the process 3702 of referring to the EqDinformation DB 601, the EqD adjustment process 3704, and a process ofreflecting the setup value to the new ONU onto the database of theentire system. These processes are necessary to set the logical distanceintegrated in the entire disposed system. Unifying logical distanceswith other disposed PON interfaces, which may not be obtained in asingle PON interface, may be done by setting the EqD according to thedistance to the ONU that has been set longest in the system. This allowsthe period of DBA control and boundary (start, end) timing of the periodto be unified, thus making it possible to perform the DBA control thatis interlocked in the entire system.

FIG. 39 is a flowchart illustrating a process of corresponding to aranging start notification form another OLT in OLT bandwidth control.When ranging information is received from another PON (OLT) (3801), theallocation band from the OLT to all of the ONUs under the control of theOLT becomes zero according to the ranging timing (3802). Next, bandallocation is conducted on the ONU under the control of the OLT in thetiming available with the ranging avoided (3803). The calculation resultis registered in the bandwidth allocation management table in the DBAcontrol unit, and the table is referenced upon generation of thedownstream frame and notified to the ONU. The overlapping of upstreamsignals may be prevented upon ranging by notifying the ranging timingbetween the OLTs. The time taken for ranging is allocated, for example,2 frames in case of G-PON having an optical fiber of 20 km (ITU-TRecommendation G 984. 3, “Gigabit-capable Passive Optical Networks(G-PON): Transmission Convergence Layer Specification”). Afternotification of ranging process is received from another OLT, allocationof upstream communication bandwidth is stopped with respect to the ONUunder the control of the OLT in response to notification timing of theranging frame released from the OLT. It is possible to avoid mutualinterference between PON interfaces to effectively control upstreambands by mutually notifying the ranging timing in advance.

The flowchart represents an order in a case where the OLT isautonomously cooperated to perform bandwidth sharing. Further, the sameorder may be used even in a case where the ranging timing is included inthe available bandwidth that is notified from the DBA control unit toeach OLT. The present invention features the process 3802 of determiningthe upstream communication stop frame from the ranging timing and theprocess 3803 of performing the bandwidth allocation using available(remaining) bands.

The upstream bandwidth control method has been described so far in acase where the same DBA period is used in the disposed PON systems.Hereinafter, a case will be described where an optical fiber is sharedby PONs each having the different DBA period.

In a case where the DBA period is different for each and every PON, eachsystem gives a bandwidth permission to the ONU at any timing. However,it is needed that the signal transmission times are not overlappedbetween systems with respect to upstream signals from the ONU. It isextremely difficult for each system to always control signal arrangementbetween the systems with respect to the upstream band allocation whilecontinuing bandwidth control at each constant DBA period in comparisonwith a case where the DBA period is unified. The following methods areconsidered to realize this.

The first method is to set the DBA periods to be an integer multiplebetween systems to be disposed. Upstream bandwidth management becomeseasy to realize by constantly maintaining between plural devicesprocessing time when a time period of collecting band requests and atime period of notifying the bandwidth allocation result by the DBA areagreed to each other. Also, since bandwidth allocation process isperformed at the constant timing, there is an advantage that it is easyto make a reservation of a band for upstream communication, in a casewhere information, for example, such as TDM signals, whose communicationtiming and communication data size are likely to be fixed for eachperiod, is included in the communication data.

FIG. 40 depicts a bandwidth allocation method upon upstreamcommunication according to a first method. An example of an upstreambandwidth use scheduling method is shown in a case where there areplural OLTs (n OLT herein) and the DBA control period is different foreach OLT. The OLT1 counts bandwidth requests of the upstream signalreceived at the slot 4211, calculates the band at the slot 4212, andstores the result in the bandwidth allocation information database. Thetable is read out at the next time range 4213 and band-notified to theONU, and the information is reflected at the next period 4214 andtransmitted as an upstream signal from the ONU. The same cycle isexecuted on the PON having the different DBA period. Here, the EqD isadjusted considering that the time when the bandwidth instruction fromthe OLT is different in response to the communication distance of theoptical fiber. For example, if bandwidth instruction transmitted to thetime slot 4213 is reflected onto the slot 4214 with respect to the OLT2and the instruction notified to the slot 4223 is reflected onto the slot4224, the operation may be continued with a constant operation ordermaintained between the systems.

In FIG. 40, plural forms of bandwidth instructions are considered thatmay be allocated from the OLT to the ONU. One of them is an instructionof the transmission time and the amount of transmission data based onthe receiving timing of the downstream frame (bandwidth instruction) inthe ONU similarly to a case where DBA period is common (refer to FIGS.26A and 26B). For this case, relative time up to 4203 is instructed, forexample, based on the period boundary 4202 of the bandwidth control. Incase of G-PON, the bandwidth is instructed to exceed the frame of 125microseconds. That is, it is specified that transmission is initiated580 microseconds after the period boundary 4202 to transmit 500 bytes.FIG. 41 shows an aspect, wherein the transmission timing 4301 instructedto the upstream bandwidth by the OLTn is allocated to cross plural basicframes of 125 microseconds. Another method is a method of instructing abandwidth on the basis of arrival of a downstream frame of 125microseconds every time. In this case, a counter for basic frames isprovided in the DBA period to identify the DBA period, and the framecounter (that is, frame identifier) and bandwidth allocation location inthe frame are notified. FIG. 42 depicts a bandwidth instruction methodfor this case. Data 4301 to be transmitted at an upstream bandwidth isdivided into 4301-1 to 4301-6, an instruction for transmitting each ofthem is divided into six, and notified to the ONU. Specifically, anidentifier (order number herein) is designated to a frame included inbetween 4202 and 4203 that are the maximum DBA periods, and thetransmission location is designated in each frame. The transmissionstart location of the division frame 4301-1 corresponds to the time 4402of the reference frame #2, and data is transmitted to the referenceframe #2 therefrom as much as transmittable. In the division frames4302-2 to 4302-6, the transmission start location is the head of eachdivision frame. In the last division frame 4301-6, the end location isin the middle of the division frame. For example, it may be instructedthat other data are transmitted thereafter.

A construction example of a bandwidth allocation information table shownin FIGS. 41 and 42 is shown in FIGS. 43A and 43B. A frame identifier4501 is added to the basic band allocation information shown in FIGS.26A and 26B. The frame identifier 4501 is notified to the ONU along withthe bandwidth allocation start time and the band allocation amount (or,bandwidth allocation start time and bandwidth allocation end time). TheONU has an upstream frame counter, and recognizes the timing whichshould be transmitted next from the information. Further, the bandwidthallocation start time stores a relative time based on the start time ofthe frame counter for each OLT (that is, DBA period boundaries 4201,4202, and 4203) in FIG. 41, and stores a relative location from thestart time of the basic frame of 125 microseconds in FIG. 42. Inaddition, the frame identifier 4501 is not needed in FIG. 41.

FIG. 44 depicts bandwidth information notified from the OLT to the ONUin FIGS. 41 and 42.

The downstream frame includes a header PDBd 4410 and a payload 4420. Theheader includes Psync 4411 which is a signal pattern for synchronizingoptical signals and US BWmap (Upstream Bandwidth Map) 4413 forinstructing bandwidth allocation at the next transmission period (DBAperiod) with respect to the individual ONU2. The other field 4412includes an Ident (Identification) field that provides a message fieldPLOAMd (PLOAM for Downstream) for ONU activation, error detection, andother control, and a super frame counter (not shown) for plural basicframes used for encryption process. Further, the header 4410 includes aPlend (Payload Length Downstream) field that represents the number ofAlloc-IDs (that is, this means providing the size of the header)instructing a bandwidth to the US BWmap.

US BWmap (4413-1 to 4413-n) includes Alloc-ID (4413-1 a), Flags (4413-1b), Sstart (4413-1 c), Sstop (4413-1 d), and CRC (4413-1 e) for eachAlloc-ID. The Sstart (4413-1) represents the bandwidth allocation starttime, that is, transmission start timing of data that is transmitted andcontrolled at the Alloc-ID. Further, the Sstop (4413-Id) is a field forinstructing the bandwidth allocation end, that is transmission endtiming.

The transmission timing is designated at the Sstart (4413-1 c) and theSstop (4413-1 d) without being bound to the basic frame length (125microseconds in GPON) to designate the bandwidth at the downstream framein FIG. 41. The upper limit of the Sstart (4413-1 c) and the Sstop(4413-1 d) is within the length included in the DBA period boundaries4202 to 4203, and this is represented by time or byte number.

In a case where bandwidth is instructed as shown in FIG. 42, the upperlimit of the Sstart (4413-1) and the Sstop (4413-1 d) becomes the timeincluded in a basic frame (125 microseconds) or a value obtained byconverting it by byte (38880 bytes in case of G-PON 2.4 Gbps). Instead,the frame number counter 4413-1 f included in the DBA periods 4202 to4203 is notified along with the bandwidth instruction as shown in FIG.44, and the upstream frame is is time-division multiple controlled asseen from the entire system.

FIG. 45 depicts a function block of an ONU for performing band controlin FIGS. 41 and 42. This differs from the conventional basicconstruction in that an upstream frame counter 4720 is provided.

The ONU2 includes an optical module 340 that terminates the opticalfiber, a PON terminating unit 330, a memory 350, an Ethernet lineterminating unit 310 that accommodates an Ethernet line 301, and a TDMline terminating unit 320 that accommodates a TDM line 302.

The Ethernet line terminating unit 310 extracts an Ethernet frame from asignal entered through the Ethernet line 301 and notifies it to the PONterminating unit 330. The Ethernet frame extracted by the Ethernet lineterminating unit 310 is stored at a data queue 351 of the memory 350.The data queue 352 is managed by a queue control unit 351 and read outaccording to an instruction notified from the upstream frame generatingunit 332 of the PON terminating unit 330 to the memory 350. Further, thedata queue 352 is reconstructed from the downstream frame received atthe optical module 340.

The Ethernet frame is stored in a data queue (transmission queue) fordownstream Ethernet frame provided in the data queue 352 of the memory350. The downstream queue control unit included in the queue controlunit 351 sequentially transmits the frame from the data queue to theEthernet line terminating unit 310 in response to a reading instructionfrom the Ethernet line terminating unit 310.

The data queue 352 may be provided in the line terminating units 310 and320, and the function of the PON-IF 300 is not affected if a primarysignal transmission line is secured at the line terminating units 310and 320. The PON-IF is a set of series of functional blocks configuredover an ASIC, and this may employ any construction as long as theabove-mentioned process may be executed.

A downlink frame analyzing unit 331 of the PON terminating unit 330extracts Ethernet (and TDM data) from the downstream PON sectioncommunication frame accumulated in a downstream frame buffer 333, andreconstructs data with the format of being transmittable from the lineterminating units 310 and 320. Further, the downstream frame analyzingunit 331 extracts device control information and band allocationinformation notified from the OLT1. The device control information isprocessed by a CPU connected to the inside or outside of the device. Thebandwidth allocation (transmission permission to individual ONU2)information to the upstream frame is retained in a bandwidth allocationinformation database 334 provided in the PON terminating unit 330. Thedatabase is referred to from the upstream frame generating unit 332, andthe transmission data amount (bandwidth allocation size) at the upstreamframe is interlocked at the queue control unit 351 and used to controlthe reading amount of the data queue 352.

A transmission control unit 4710 is provided to transmit an upstreamframe based on the bandwidth allocation information database 334. Thetransmission control unit 4710 has timings when a bandwidth controlboundary comes and a frame counter among them, and an upstream frame istransmitted by a combination of the timing and information from thebandwidth allocation information database 334. The case of FIG. 42includes the frame counter shown in FIG. 44 as bandwidth allocationinformation in addition to the case of FIG. 41. Accordingly, theinformation included in the bandwidth allocation information database334 slightly differs between FIG. 41 and FIG. 42. In FIG. 41, the headerprovided to the data may be one even in a case where there are many dataamounts because data is transmitted over plural frames. In contrast, themethod shown in FIG. 42 needs to insert a header to a divided piece ofthe data when the frame is divided, and therefore, the method shown inFIG. 41 is more advantageous in terms of the use efficiency oftransmission band. On one hand, in the case shown in FIG. 41, data frameis longer than physical layer frame (basic frame, that is, G-PONEncapsulation Method (GEM) frame in G-PON), and there is demanded astructure of managing the division number and frame length with respectto division upon transmission of the data and reproduction upon receiptover the basic frame. Since the entire data frame is completed in thebasic frame in FIG. 42, frame generation and termination may be easilyrealized by existing structure.

In the exemplary embodiment of FIG. 41, the upstream frame counter inthe ONU is reset at the reference period that is shared at the entiredisposed systems, that is, at the time of DBA period boundaries 4201 and4202. The counter is used to count the relative number of frames fromthe DBA period boundaries 4201 and 4202. It may be possible to suppressthe amount of information upon transmission of the downstream framesince it is enough to notify only the amount of data transmission (bytesor bit number) and frame counter values that correspond to existing 125microseconds when transmission time slot is designated from the OLT tothe ONU to exceed the frame of 125 microseconds by using the framecounter. This is possible in performing a transmission timinginstruction. The same effect is also true for the frame counter shown inFIG. 42. On the contrary, in the method of directly designating arelative location from the reference period boundary (DBA periodboundaries 4201 and 4202 herein) without using the frame counter, theamount of data transmittable is large in comparison with one frame of125 microseconds, and therefore, lots of bit numbers are needed todesignate the transmission location.

Meanwhile, expanding the bit numbers designating the transmission startand the end location may be realized by the conventional structure.

The second method is a method of controlling upstream bandwidth atindependent timing by each of disposed systems. This requires executingreservation of an upstream bandwidth while identifying conditions ofanother upstream bandwidth every period. A function necessary hereinincludes priority setting (described with reference to FIGS. 35A and35B) for determining reservation order of an upstream bandwidth, areference timing for identifying a bandwidth used by another system, andsetting of a reference frame. The latter is a parameter that should becommonly determined in all of the OLTs in the system separately from theDBA period for each system, and the individual PON may map its DBAcontrol timing and period with respect to the reference period by usingthis.

FIG. 46 depicts a bandwidth allocation method upon upstreamcommunication according to a second method. There is illustrated anexample of a use scheduling method of an upstream bandwidth in a casewhere plural OLTs (n OLTs herein) exist and the DBA control period isdifferent in each OLT. An operation of OLT 1 and OLT 2 is as shown inFIG. 40. It is shown herein that upstream communication timing from eachONU managed by the OLT 1 and the OLT 2 is different. As mentioned above,EqD adjustment between the OLTs considering the communication distanceof the optical fiber is not necessary in the second method.

Even in FIG. 46, the form of bandwidth instructions that may beallocated from the OLT to the ONU may be provided in a plural. One ofthem is an instruction of transmission time and transmission data amountbased on the downstream frame (bandwidth instruction) receiving timingin the ONU similarly to a case where the DBA periods are common (referto FIGS. 26A and 26B). In this case, there does not exist a commonperiod boundary unlike FIG. 40. Accordingly, the boundary of referencetiming used, for example, as a common frame counter is shared by theentire OLTs, and a relative time is designated for that time. Forexample, in case of G-PON, the bandwidth is instructed to exceed 125microsecond frame. That is, such a designation is performed that thecommon counter boundary 4801 or 4802 is set as the zero point andtransmission is started after 580 microseconds to transmit 500 bytes.FIG. 47 represents this aspect. The transmission timing 4301 instructedto the upstream bandwidth by the OLTn is allocated to cross plural 125microsecond basic frames. Another method is a method of instructing aband based on arrival of a downstream frame of 125 microseconds everytime. In this case, a counter for basic frames is provided in the DBAperiod to identify the DBA period, and the frame counter (that is, frameidentifier) and bandwidth allocation location in the frame are notified.FIG. 48 depicts a bandwidth instruction method for this case. Data 4901to be transmitted at an upstream bandwidth is divided into 4901-1 to4901-6, an instruction for transmitting each of them is divided intosix, and notified to the ONU. Specifically, an identifier (common framecounter herein) is designated to a frame included in between 4802 and4803 that are the common frame counter periods, and the transmissionlocation is designated in each frame. The transmission start location ofthe division frame 4301-1 corresponds to the time 5002 of the referenceframe #2, and data is transmitted to the reference frame #2 therefrom asmuch as transmittable. In the division frames 4902-2 to 4902-6, thetransmission start location is the head of each division frame. In thelast division frame 4901-6, the end location is in the middle of thedivision frame. For example, it may be instructed that other data aretransmitted thereafter.

A construction example of a bandwidth allocation information table shownin FIGS. 47 and 48 is shown in FIGS. 49A and 49B. The table constructionis as shown in FIGS. 43A and 43B. In FIG. 45, the bandwidth allocationlocation 5101 represents a relative time from the start time of thecommon frame counter, and does not require the frame identifier 5102. InFIG. 48, the bandwidth allocation location 5101 is a time from the starttime of a basic frame of 125 microseconds and this is updated everybasic frame. Also, the frame identifier 5102 is a counter of a referenceframe, and is notified to the ONU together with the band allocationstart time and the bandwidth allocation amount (or, bandwidth allocationstart time and band allocation end time). The ONU has an upstream framecounter, and recognizes the timing which should be transmitted next fromthe information.

The bandwidth information notified from the OLT to the ONU is as shownin FIG. 44. What is indicated by each field is different as describedabove with reference to FIGS. 49A and 49B.

The construction as shown in FIG. 45 may also apply to the ONU functionblock. In FIGS. 49A and 49B, the frame counter becomes a counter of thecommon frame.

Since data is transmitted over plural frames in FIG. 47, the headerprovided to the data may be one even though there are lots of dataamounts. On the contrary, the method shown in FIG. 48 may insert aheader into a division piece of the data when the frame is divided, andtherefore, the method of FIG. 41 is advantageous in terms of useefficiency of transmission bandwidth. In the meanwhile, in the caseshown in FIG. 47, data frame is longer than the physical layer frame(basic frame, that is, GEM frame in G-PON), and a structure is in demandwhich can manage division numbers and frame length over the basic framewith respect to division upon transmission of the data and reproductionupon receipt of the data. Because the entire data frames are completedin the basic frame in FIG. 48, frame generation and termination may beeasily realized by the structure according to the existingrecommendations.

In the exemplary embodiment of FIG. 47, the upstream frame counter inthe ONU is reset at the reference period shared by the entire disposedsystems, that is, at the time of DBA period boundaries 4801 and 4802.The counter is used to count the relative number of frames from the DBAperiod boundaries 4201 and 4202. It may be possible to suppress theamount of information upon transmission of the downstream frame since itis enough to notify only the amount of data transmission (bytes or bitnumber) and frame counter values that correspond to existing 125microseconds when transmission time slot is designated from the OLT tothe ONU to exceed the frame of 125 microseconds by using the framecounter. This is possible in performing a transmission timinginstruction. The same effect is also true for the frame counter shown inFIG. 48. On the contrary, in the method of directly designating arelative location from the reference period boundary (DBA periodboundaries 4801 and 4802 herein) without using the frame counter, theamount of data transmittable is large in comparison with one frame of125 microseconds, and therefore, lots of bit numbers are needed todesignate the transmission location.

Meanwhile, expanding the bit numbers designating the transmission startand the end location may be realized by the conventional structure.

The first method may employ any one of intensive control of performingband allocation in the DBA control unit, two-step control of combining aDBA process for each OLT and adjustment between the OLTs in the DBAcontrol unit, and a distribution control of autonomously transferringbandwidth reservation information between the OLTs to perform bandreservation according to the OLT priority.

The system construction for the intensive control is identical to thatwhen the DBA periods have been unified. Bandwidth allocation iscalculated at the DBA period set for each OLT while upstream bandwidthuse conditions are managed in the DBA control unit. The systemconstruction in the two-step control is also identical to that when theDBA periods have been unified. The communication data amounts andcommunication timing (upstream bandwidth allocation location) that areavailable for each OLT are arranged and managed in the DBA control unit,and the bandwidth allocated to the ONU under the control of each OLT ismanaged for each OLT. The system construction in the distributioncontrol is also equal to that in case of a single DBA period.

A key for realizing the above operations is a managing method ofbandwidth allocation locations.

In a case where the DBA period is a single one, the entire OLTs performthe DBA control at the same period and the same timing, and therefore, acommon reference may have been provided such as the arrival time of thedownstream frame which is a reference of determining the upstreamtransmission timing, or transmittable period of the upstream bandwidththat is an object of the bandwidth allocation on the basis of that time.It is preferable to unify the DBA period boundaries in the system toeffectively apply the first method. In this case, each OLT has a counterthat manages the number of frames in its own DBA period. Simultaneously,it is effective when the OLT has a frame counter that operates at thelongest DBA period so that it is used for identifying the systemsynchronization. By the information, the OLT may perform the bandwidthallocation on the ONU under its control while determining the allocationlocation of the upstream bandwidth reserved by another OLT.

The second method may be also realized by the whole forms, however, thedistribution control may be considered to be most appropriate. In thiscase, the timing control which is a reference in the entire system isperformed independently from the DBA period owned by each OLT.Therefore, a method may be used that distributes the counter long enoughnot to overlap in the system to all of the OLTs as a reference counter.When each OLT receives a reference counter transmitted from one of thedisposed OLTs or DBA control unit, the OLT matches the counter with itsDBA period and timing, and conforms with the upstream bandwidth used byanother OLT in the system.

FIGS. 50A and 50B depict a construction example of a bandwidth usagecondition management table retained in the DBA control unit in a casewhere the DBA period is different for each OLT in the first and secondmethods. The basic construction is identical to that shown in FIGS. 30Aand 30B. A common counter 5101 is added as the bandwidth allocationlocation information to grasp the upstream bandwidth use timing betweenthe OLTs. In a case where a frame counter is necessary in the firstmethod (FIG. 44), the frame number (upstream frame number including thebandwidth allocation location) counted from the time of the DBA periodboundaries 4201, 4202, and 4203 is stored. In the second method, methodsof FIGS. 47 and 48 all require a common counter. For this case, acounter value common in the system is stored in the frame counter 5101,and notified to each OLT as the bandwidth allocation information.

Since each of the DBA periods becomes an integer multiple of the firstmethod, the period of calculation and notification of the bandwidthallocation amount by the DBA is synchronized in the entire system.

Accordingly, it is relatively easy to design the system since it is easyto determine the use conditions of the upstream bandwidth and systemoperation patterns (overlapping method of the bandwidth allocationcalculation) are limited.

In the second method, the DBA period or another timing is different foreach OLT. Therefore, upstream bandwidth allocation conditions aredetermined from the upstream bandwidth information in which each OLT ismapped to the reference clock by supplying the reference clock and thereference period to each OLT.

Although the sharing algorithm of the bandwidth information isrelatively complicated, the individual OLT may operate at the freetiming (free DBA calculation period).

Since the first method requires the upstream transmission start timingon the ONU side, the first method requires the EqD to be adjustedbetween the systems like when the DBA period has been unified.

That is, in a case where the same transmission instruction istransmitted to the ONU by the entire systems (OLTs) at the same time,the EqD should be adjusted so that the upstream communication dataobserved over the trunk optical fiber is overlapped with the sametiming. The adjustment method and effect of the EqD is equal to a casewhere the DBA period has been set identically.

The second method does not have to consider the EqD of the other system.However, when the DBA operation is mapped to the reference counter, theEqD is set so that the upstream signal from the ONU may be observed overthe optical fiber at the timing of corresponding to mapping.

Accordingly, in the first method, a ranging method upon addition of anew ONU may employ the same method as a case where the DBA period hasbeen set equally. The second method compares the EqD initial value thatis a general ranging result with the receiving timing requested from themapping information and then determines the corrected value of the EqD.

FIG. 51 depicts a flowchart of the ranging process in this case. The OLTperforms ranging on the ONU under its control and obtains the longestone, that is, the distance that should be considered a reference of EqDcalculation in the conventional ranging process (5301). Next, the OLTidentifies mapping information with its DBA period setup and framecounter therein, and, as necessary, system common counter (5302) anddetermines response time (frame number) from the ONU corresponding tothe bandwidth allocation from the OLT. If it is determined EqDadjustment is necessary as the comparison result (5303), the correctedvalue is yielded 5304 and EqD correction is notified to each ONU (5305).

The other operations are equal to those shown in FIG. 38.

In disposed systems where the downstream wavelengths are common and theupstream wavelengths are different, a PON downstream frame is sharedbetween the disposed systems.

The basic construction of the system is as shown in FIG. 1, and only thewavelength allocation is different (FIG. 52).

In the system, the downstream signal from the OLT-A to the ONU under thecontrol of the OLT-A uses the same wavelength for all of the PONinterfaces. In the drawings, the wavelength is represented as λd. Withrespect to the uplink signal, the ONU transmits a signal to the OLTusing the different wavelength for each PON interface (OLT), forexample, at the wavelength λu(A) in case of the ONU under the control ofthe OLT-A and at the wavelength λu(B) in case of the ONU under thecontrol of the OLT-B. The upstream signal from all of the ONUs isdistributed to the OLT-A and the OLT-B at the same strength. The signalfrom each PON may be split from a fact that there is provided on the OLTside a filter for cutting the other wavelengths than the upstreamwavelength used for each PON and there is provided near the WDM in FIG.52 a spectral device for distributing destinations for each wavelength.Meanwhile, the downstream signal multiplexed by the WDM 120 istransferred to the ONU through the optical splitter 150 (this becomes anoptical coupler for upstream signals) and branch line optical fibers101-A-1 to 101-A-NA and 101-B-1 to 101-B-NB, each of which is connectedto each ONU. The light distributed in all directions by the opticalsplitter arrives at each ONU at the same time. Therefore, the ONU comesto receive the signals from all of the OLTs. The ONU determines whetherit is necessary information or not, for example using Port-ID, VLAN tag,or MAC address, header for PON section, L2 header, L3, or otheridentifiers included in the upper header.

It is considered that the optical fiber 100 is shared by plural OLTs.The description will be made using the OLT-A and OLT-B in the followingexemplary embodiments. However, although the number of connection of theOLTs, that is, the number of PONs sharing the optical fiber 100 isincreased, this may be applied without losing the features of thepresent invention. In this case, OLT connection optical fibers 112 and113 are used for connection of still another OLT. Also, branch lineoptical fibers 102 and 103 are used in case of adding or moving ONUsmanaged by the above-mentioned new OLTs or ONUs managed by the OLT-A orOLT-B.

FIG. 53 depicts a downstream communication method using a time-divisionmultiplexing method which is necessary to operate the system shown inFIG. 52. In the system shown in FIG. 52, all of the OLTs use the samewavelength for downstream signals from the OLT. Since it is impossibleto identify the signal from each OLT by its wavelength in the ONU, whichis on the receiving side, the signal is identified based on variousheader information included in the frame (refer to the descriptions ofFIG. 52). The OLT performs time division multiplexing to startcommunication using the frame (GEM frame in GPON) including anidentifier (Port-ID of the GEM header in GPON) for each destination inthe downstream frame in order to conduct communication with plural ONUs.Herein, time-division communication is carried out with respect tocommunication with the whole ONUs sharing a single optical fiber. Thisapplies to a case where there exist plural OLTs and each ONU isaccommodated in different OLT.

In the conventional system, the OLT having different upstream wavelengthhas managed the communication conditions with only the ONU under itscontrol when allocating a communication time to an individual ONU.Accordingly, the system should avoid the upstream communication timeoccupied by another OLT to prevent signal overlapping when band controlis performed by an OLT.

FIG. 53 is a view illustrating a frame multiplexing method over a fiberupon transmission of a downstream frame from OLT 1 to ONU 2. FIG. 53shows a state where a frame is transmitted from the right side to theleft side. In the left part of FIG. 53, data transmitted earliest isrepresented. FIG. 53 also shows an example of an arranged state offrames which are transmitted late from the OLT 1 toward the right side.The dotted-line represents a basic frame period (for example, 125microseconds).

The frame transmitted from each OLT 1 passes through the WDM and ismultiplexed into a single fiber. In the drawing, 4101-1 to 4101-n,respectively, refer to transmission locations and sizes of the fixedbandwidth communication data transmitted toward ONU#1 to ONU2#n. Theframe 4102 to frame 4107 refer to variable bandwidth data transmitted toeach ONU 2. The variable bandwidth data is inserted to the OLT not tooverlap the fixed bandwidth data upon multiplexing. Plural systems maybe used over the same fiber even in a case where the same wavelength isused for downlink by transmitting a downstream frame in a time-divisionmultiplexing method as shown in FIG. 53. This method may share anoptical module in both new and old systems, and if this function isfurther prepared, this function may be used, for example, upon upgradeof the same system.

The clock synchronization method in the disposed PON systems isassociated with control clocks of optical signals and circuits, but doesnot relate with allocation of wavelengths for communication.

Accordingly, this may be realized in the same methods as shown in FIGS.3 to 13.

FIG. 58 depicts a clock synchronization method in case of downstreamwavelength sharing. When clocks are monitored with respect to a systemthat feeds back downstream signals from the OLT, a strength identifier5850 is provided to identify the oscillation source OLT based onstrength instead of the WDM 850 that has split signals for eachwavelength in FIG. 8.

It is adapted to be capable of extracting the phase of a clock based onstrength when signals are fed back by transmitting signals each havingdifferent strength in the stage of clock synchronization for each OLT.An example of a transmitted signal is shown in FIG. 59. The referencenumeral 5910 refers to a transmission signal from the OLT-A, and thereference numeral refers to a transmission signal from the OLT-B. Sincethere is a difference between the signal strength 5931 of the OLT-A andthe signal strength 5932 of the OLT-B, the phase of clock may beproduced from the summed value of strength when optical signals arereceived.

In a case where downstream wavelengths are the same, accuracy may beimproved by adapting clock signals from the ONU side as signals on thehigh-speed side in addition to synchronizing clocks transmitted from theOLT.

In descriptions regarding a case having different wavelengths, forexample, ONUs for 10 Gbps transmission are operated using clock signalsof the OLT for 10 Gbps to control clocks of ONUs for 1 Gbps using clocksignals of the OLT for 1 Gbps. As mentioned above, it is important toaccurately adjust location of a ranging reference point in thePON-disposed system. Here, difference in clocks of the ONUs for 1 Gbpsis reduced by using 10 Gbps clocks for synchronization of the ONUs for 1Gbps.

1 clock variation of 1 Gbps clocks may cause 10 clock variation whenbeing recalculated by 10 Gbps clock conversion. Accordingly, guide bitsetup may be made small considering clock variation of low-speed signalsby using high-speed clocks even for synchronization and monitoring oflow-speed clocks, and this leads to improvement in bandwidth usageefficiency as a whole. In this case, the ONU internally generates periodsignals for low-speed clocks for loading low-speed clocks ontohigh-speed clocks to map 1 Gbps signals to 10 Gbps clocks. When 1 Gbpsdata signals are loaded onto 10 Gbps clocks, signals are loaded on the 1Gbps signals generated based on 10 Gbps signals inside the ONU andoscillated. And, the delimitation of generated 1 Gbps signals areadjusted to conform to timing of 1 Gbps period signals that come outbefore being notified together with 10 Gbps signals.

A construction of the ONU is shown in FIG. 60. A synchronization controlunit of the ONU is included in an optical module and a CDR unit (340 inFIG. 45). When receiving clock signals through a clock receiving unit6010 (this may be considered as an interface with the optical module orthe function of the optical module itself is also considered included)and an optical device, the PON control unit 6000 performs a phaseextraction process and signal synchronization for clock reproduction.The clocks obtainable herein are then divided to be utilized asin-device clocks. In the exemplary embodiment, next, a low-speed clockgenerating unit 6030 generates low-speed clocks and a clock comparingunit 6040 conducts phase comparison with high-speed clocks. Phaseinformation obtained herein is transferred to a bit buffer 6050. The bitbuffer 6050 adjusts timing of low-speed clocks and high-speed clocksfrom the phase information. The low-speed clocks obtainable as a resultare sent from the clock transmission interface to the device. Thefunctions of the transmission interface may be included in the bitbuffer, and this is not necessary to provide separately. This alsoapplies to any drawings of the present invention. Although it has beendescribed to generate clocks for device at low-speed clocks, the clocksfor device may be generated at high-speed clocks.

Since wavelengths are different for each PON interface on the OLT sidein a case where only downstream wavelengths are the same, each OLT mayperform DBA control at any period and timing, and it is unnecessary tounify the logical distance for all of the disposed PONs. However, it ispreferable to set the logical distance to have more margins than generalPONS since there is a need of determining (adjusting timing for timemultiplexing transmission) transmission timing of downstream frames thatnotify uplink band allocation although the upstream frame transmissionitself is performed at each different wavelength.

Two methods are considered for methods of transmitting downlink frames.One of them is to share headers of the entire downstream period framesand transmit information by time multiplexing data in the payload. TheGPON shares PCBd headers (representative OLT is transmitted) andtime-multiplexes timing when a GEM frame is inserted into the payload.For example, in case of G-PON, upstream communication (response) timingfrom an ONU is determined on the basis of arrival time of downstreamperiod frame header PCBd. Accordingly, downstream bandwidth may beeffectively used without a need of transmitting the same header from theentire OLTs. At this time, upstream band control information which is aninstruction for ONU 2 included in the header is notified to therepresentative OLT that transmits the header in order to share theheader in the entire OLTs.

FIG. 54 depicts a downstream frame transmission method when the entireheaders of downstream period frames are shared. In this drawing, amethod of configuring downstream frames will be described taking framesof GPON as an example. FIG. 54 shows a downstream frame constructionitself of G-PON. Accordingly, the other parts than that associated withthe exemplary embodiment will be excluded from descriptions.

The entire PON systems sharing the optical fiber utilize headers 5410-1and 5410-2 (hereinafter, referred to as “5410”) of a downstream frame5400. Among them, PLOAMd 5413 and US BWmap 5460 include a message and abandwidth instruction from plural OLTs but not a message and a bandwidthinstruction from a single OLT like in the prior art. In addition, thePLOAMd 5413 is a field used for startup of ONUs, allocation of ONU-ID orAlloc-ID, or control of distance under operation or monitoring oferrors. The US BWmap 5460 is used to notify timing of transmittingupstream frames to an individual ONU, and plural sets are generallyinserted into this field, with a field group of 54611 to 54615 as oneset. The Alloc-ID 54611 used herein is a parameter specifying the unitfor bandwidth control, and this is allocated more than one for each ONU2. Each OLT 1 notifies a bandwidth instruction for the ONU 2 under itscontrol (that is, information to be inserted into the US BWmap field5460) to the representative OLT that transmits the header. With respectto communication between OLTs, an effective frame format may bespecified only in the device similarly to the inner frame, or this maybe transferred using existing protocols such as Ethernet. There are lotsof methods with respect to mounting, and therefore, using any methoddoes not affect the spirit of the present invention.

One or plural frames (GEM frame in G-PON) is inserted in the payload5420 of the downlink frame 5400. This frame is transmitted from each OLTto an ONU under the control of the OLT. Accordingly, in a case where anOLT transmits a downstream frame, the OLT divides transmission timingbetween OLTs and transmits the frame so as not to overlap a downstreamframe transmitted from another OLT.

The detailed method has been already described above, and therefore,repetitive descriptions will be omitted. A frame of an individual ONU2includes PLI (Payload Length Indicator) representing frame length,Port-ID for identifying a destination ONU 2, PTI (Payload TypeIndicator) for identifying the type of information in the frame (forexample, such as whether the information is data for maintenance oruser's data), and an HEC field 54214 used for error correction code(ECC) of the header 5421 h. The individual ONU2 may extract informationtherefrom of the downstream frame by allocating Port-ID to theindividual ONU2 so as not to overlap between the entire systems sharingthe optical fiber.

Upon transmission of frames in FIG. 54, (taking GPON as an example),transmission resource OLTs are different in the boundary of GEM frame.Since clocks are unified according to the above-mentioned clocksynchronization method upon transmission of downstream signals, clocksmay be expected to be synchronized with each other at a certain degreeupon receipt in the ONU 2. By doing so, phase identification may beconducted for each GEM frame, and this may expect further stabilizedoperations. The following functions are provided to identify phase.

In the G-PON, the frame synchronization is identified according towhether HEC included in the header of a GEM frame may be taken or not,and in addition thereto, clock phase information is retained in the ONUside for each OLT number. This lets the OLT have a function formemorizing a clock phase (eye pattern receiving timing) when the signalsfrom the ONUs are synchronized on the OLT side. Specifically, the amountof shifts in phase is maintained in the receiving unit to absorb theshift of clock phases when one clock signal is extracted. The method isnot limited, however, there is a method of catching a location havingoptimal receiving sensitivity as the phase of the data by receiving asignal at plural receiving locations where a tiny amount of phases havebeen shifted compared to the phases of data signals upon achievingsignal synchronization. In the G-PON, this method is used upon receiptof 1.2 G upstream signals. It removes any necessity of comparing andselecting plural receiving patterns for each data frame receipt toretain the optimal location obtainable in this case, thus making itpossible to reduce the probability of losing synchronization.

Another method is to transmit a downstream frame from each OLT in thecomplete form and control transmission timing between OLTs so thatsignals do not overlap each other. For this case, it is not necessary tounify operation periods between OLTs. This may be used for a case whereoperation clocks for OLTs are different or OLTs having differentinformation processing efficiency therein are disposed. Further, sincethe timing for TDM may be dynamically varied by communication betweenOLTs, this is effective, for example, in a case where plural PONs aredisposed that have different frame formats such as G-PON and GE-PON.Because of not being specified, the timing for transmitting downstreaminformation between OLTs is notified by the OLT even in this case, andthere is required a structure that avoids overlapping between signals.

FIG. 55 depicts a downlink frame transmission method when each OLTtransmits a downlink frame in the complete form.

In this case, plural PCBd header 5510 added frames are inserted in thebasic period 5500 of the downstream communication control (125microsecond period in G-PON). The difference from FIG. 54 lies inwhether the header 5510 is shared by the entire OLTs or separatelytransmitted. In case of FIG. 55, when the frame synchronization patternis inserted before the header 5410-2, a process of synchronizing anindividual frame in the ONU2 may utilize the conventional technology. Asanother method, in a case where clock synchronization accuracy issufficient, start location of the header 5510-2 of the subsequent framemay be determined by referring to information on frame length 5515 a,5515 b, and 55211 included in the header 5510 without inserting theframe synchronization pattern. In the latter case, phase shift ofdownstream frame may be suppressed by preparing phase identificationfunction of downstream frame as in FIG. 54, and therefore, stableoperations may be done even though the number of the OLTs has beenincreased.

The basic construction of the header 5510 is as shown in FIG. 54.However, only the information on the ONU which is managed by individualOLT is inserted into the US BWmap 5560. Accordingly, header constructionand content of the downstream frame are identical to the construction ofthe header that has been used for existing PON at the beginning of GPON.The ONU2 may refer to the US BWmap 5560 of the downstream frame andPort-ID 55212 included in the header 5521 h of the data frame todetermine whether it should gather the data (bandwidth instruction anduser's data).

In a case where downlink frames are common, it is necessary to providean ID for band allocation to identify a specific ONU with plural PONsystems disposed. There are plural implementing methods. One of them isa method of using independent identifiers (Alloc-ID in case of GPON)with respect to the entire ONUs sharing the optical fiber. This methodhas a merit of being capable of the conventional frame format withoutany variation. In a case where the number of band allocation IDs isrelatively small, a request may be satisfied which reduce developmentcosts and maintains operation efficiency. In another method, anidentifier for identifying the PON system is separately provided fromthe conventional bandwidth allocation ID, and the bandwidth allocationprocess is performed like the prior art in the individual PON system.Because the number of independent band control IDs may be small in thiscase, the size of the memory in the individual device and logicalcircuits may be reduced, thus resulting in lowering in costs. Becauseoperation such as searching becomes fast as the number of the bandcontrol IDs, improvement in performance may be expected.

In a case where upstream wavelengths are different for each ONU, at thistime they are not applicable, for example, overlapping between signalsin upstream communication. It is unnecessary to adjust the responsedelay time of the ONU between systems that has been performed whenupstream wavelengths are the same without TDM control.

The setup of EqD may be carried out independently for each PON system.The process at the startup of ONU sets the logical distance necessaryfrom the OLT side by the transmission timing of the downstream frame. Itis required to adjust the transmission timing of the downstream frame byreferring to a common frame counter for controlling the downstreamcommunication and making the boundaries of the DBA control periodsunified with respect to the OLT (at the beginning of the OLT). However,if the adjustment is complete at the OLT side, the subsequent ONU startprocessing may be performed by using the conventional method, and theranging process and its result, i.e. determination of EqD are notaffected by the other systems.

In a case where the upstream and downstream wavelengths are set to becommon, a clock synchronization method between the OLTs may be realizedby the same method as described above with reference to FIGS. 1 to 13without the aid of setup of wavelengths.

Such a process as described above with reference to FIGS. 14 to 51 isrequired for DBA control and setup of logical distance that is neededfor control of upstream communication timing. Also, adjustment oftransmission timing is necessary between the OLTs and such a process asdescribed above with reference to FIGS. 52 to 56 is performed withrespect to the downlink communication timing.

1. A network system comprising: a first OLT (Optical Line Terminal) anda second OLT; a first ONU (Optical Network Unit) and a second ONU; and aclock synchronization control unit connected to the first OLT and thesecond OLT to supply a common clock timing, wherein the first ONU andthe second ONU are connected to each other through an optical fibershared by the first OLT and the second OLT such that data is transmittedbetween the first OLT and the first ONU and between the second OLT andthe second ONU, wherein each of the first OLT and the second OLTincludes: a clock oscillator which generates a clock signal; a clockreceiving unit that receives the common clock timing from the clocksynchronization control unit; and a clock detecting unit thatsynchronizes the clock signal generated from the clock oscillator withthe common clock timing received by the clock receiving unit, andwherein the first OLT and the second OLT supply the clock signal to thefirst ONU and the second ONU, respectively, at the common clock timing.2. The network system according to claim 1, wherein the first and secondOLTs perform DBA (Dynamic Bandwidth Assignment) control using the commonclock timing.
 3. The network system according to claim 1, wherein thefirst and second OLTs determine timing by performing ranging using thecommon clock timing.
 4. The network system according to claim 1, whereincommunication between the first OLT and the first ONU and communicationbetween the second OLT and the second ONU differ in bit rate from eachother.
 5. The network system according to claim 1, wherein a bit rate ofcommunication between the first OLT and the first ONU is an integermultiple of a bit rate of communication between the second OLT and thesecond ONU.
 6. The network system according to claim 1, wherein thefirst OLT and the first ONU perform communication at a higher bit ratethan that of the second OLT and the second ONU.
 7. A network systemcomprising: a first OLT (Optical Line Terminal) and a second OLT; afirst ONU (Optical Network Unit) and, and a second ONU, where the firstONU and the second ONU are connected to each other through an opticalfiber shared by the first OLT and the second OLT such that data istransmitted between the first OLT and the first ONU and between thesecond OLT and the second ONU; a clock synchronization control unitconnected to the first OLT and the second OLT to supply a common clocktiming, where the first OLT and the second OLT supply a clock signal tothe first ONU and the second ONU, respectively, at the common clocktiming; and a feedback line that inputs to the clock synchronizationcontrol unit the clock signal supplied from the first OLT to the firstONU and the clock signal supplied from the second OLT to the second ONU,wherein the clock synchronizing control unit includes: a clock comparingunit that compares the clock signal received from the feedback line withthe common clock timing generated by the clock synchronization controlunit; and a clock correcting unit that corrects the common clock timingbased on a comparison result.
 8. A network system comprising: a firstOLT (Optical Line Terminal) and a second OLT; a first ONU (OpticalNetwork Unit) and a second ONU; and wherein the first ONU and the secondONU are connected to each other through an optical fiber shared by thefirst OLT and the second OLT such that data is transmitted between thefirst OLT and the first ONU and between the second OLT and the secondONU, wherein the first OLT includes: a clock oscillator that generates acommon clock signal, and an interface that supplies the common clocksignal to the second OLT; and the first OLT and the second OLT supply aclock signal to the first ONU and the second ONU, respectively, at aclock timing of the common clock signal that is supplied from theinterface.